Now that I have been mostly playing Warlock ever since Eldar came out (I didnt play as other factions since then), I can humbly share some experiences and observations with regard to playing as this class of Eldar.
Bear in mind this experience is based on a current stage of the game (July 2016), so I'd imagine some of these might expire with development of the game.

1. KEY TO VICTORY IS "SEER COUNCIL"
Now I know Warlocks are not Seer Council from tabletop variant, but the game sure looks like it when you have a good bunch of Warlocks swarming one point
I believe that when it comes to Eldar and their numbers on the field, this is a rough balance that should be kept with regard to the number of Warlocks.

5-8 Eldar = 1 Warlock
10-12 Eldar = 2-3 Warlocks
15-20 Eldar = 4-5 Warlocks
22+ Eldar = 6-8 Warlocks

They seem to perform best in this equation in my opinion in order to keep healing and helping the team as well as leaves plenty of space for other classes.

1A. BALANCING THE TEAM
If anything the Eldar are and will be even more varied in classes (5 classes now, 2 more to come apparently) than their enemies, so as Warlock as well as Eldar player when coming onto the field it is very important to see and judge the amount of deployment of classes in the current match.
If all classes are present on the field then you can pick the one you like but if you see that there is a lack of some class or a certain disbalance - for example 1 Dire Avenger in a team of 12 - then you should probably pick Dire Avenger.
Too many Warlocks is also a problem because it is all good and well that everyone is alive but you often end up in a situation when you have 3 of them on a point and no Dire Avenger around to actually capture the point - this is a shameful thing happening and it is a current weakpoint in Eldar gameplay for either bad coordination of the team/squad or we simply lack that 1 extra class to capture.
So concluding this point it is important to see whos who first and make a good judgement of a class balance before blindly picking your current favourite class.

2. Defensive Warlock
*This point is not about game mode but a playstyle of the Warlock class, that can be applied in any kind of situation*
Being a defensive Warlock is perhaps easier side of this class.
You stick to bunch of Eldars that are either defending or advancing, and you try to keep everyone alive.
You always stay behind your kin and try not to expose yourself, look for hide spots and good cover where you can heal your
team mates from. You heal and buff the team in combat but you are on the far end of it and you do not engage unless you are one or one
of the very few left standing.
If you are buffing some dark reapers however, who can't really fight back that well in melee and their agility is crippled, it is your sole duty to defend him in melee combat before a charging enemy can get to him/her.
It is common practice to flank or sneak upon anti vehicle/suppression class so as a healer in a situation when there is no other class around it is your responsibility to watch their backs at the cost of cutting off the healing stream.
A defensive Warlock will keep the juice running to anyone around him before engaging in direct combat - keep to the ledges and be a beating heart of the team.

3.Offensive Warlock
Warlocks are excellent blade masters with fearless agility and psychic will to strike down their foes with their witchblades.
If you ever find yourself in grave danger or find the urge to go lone wolf, here are some of the Warlock specs and tips in combat.
You clear about 8 to 9 meters with charged attack.
Your flips clear equal 8 meters on a straight line.
You cover about 4 to 5 meters forward with your normal single strike.
Warlock flips are an essential tool to get to cover, especially in enclosed areas such as corridors, halls and capture points.
When you are positioned right, a single flip can land you fully in cover and get you that point of initiative of what to do next - recover and engage, run towards team mates or run for your life - before an enemy will get to you.
A flip can always save you from enemy fire, grenade, a heavy melee strike and even vehicle fire!
In direct combat, it is a great tool to reduce the final distance when you are about to strike your enemy, or an evasive tool when running between cover.
The witchblade is a mighty weapon, but it doesn't deliver as powerfull strikes as other factions melee weapons, so especially in 1v1 situation you will find that you have to cash in on a good few strikes - usually 3 to 4 on an ordinary unit and 5 to 7 on a heavily armoured unit (ground assaults) of normal strikes and heavy strikes 2 to 3 on most units and for ground assaults I honestly didnt down one yet in a 1v1 situation where there was no interference from either of sides with just heavy strikes, it was either normal strikes all the way or combination of heavy and normal.
Warlocks swing rather fast but make sure to throw in that defensive bash in your combination of moves to give you an opening for your next move, either strike, dash, or dash and flee.
Which brings me to a point of asking: What is the best melee combination?
In my opinion from what I've seen in current gameplay, I honestly couldn't pin down a certain combination that would win me every fight, maybe someone already has? I don't know.
I noticed this though - a single move will almost always get you dead. Meaning that if you just swing normal attack all the time and pray for a miracle, it very very very rarely happens.
When I enter combat, I usually try to punch in a heavy attack or if the situation is more hairy (more enemies around), I tend to attack from run or flip and normal attack.
In a scenario of mayhem (when everyone is around, the place is packed and we need to break their ranks) I tend to swing in or flip in, hoping that I got a good defensive Warlock behind me and I can down as many enemies as I can by doing a constant combo of flips and attacks, with a rare use of defensive bash - this only happens when I find myself engaged in melee combat and enemy isnt reinforced with ranged fire, otherwise there is no point in hanging around when a power axe is supported with a boltgun or two, you need to dash and attack randoms if you hope to down anyone in an advancement situation as offensive Warlock.
When 1v1 however I stick mostly to normal attacks and defensive bashes, with a very occasional heavy attack and flip.
What I noticed is that when you throw in a flip in direct combat you can throw off enemy from their balance and timing of strikes/moves.
But this move can sometimes backfire horribly when against a good player, because as soon as you touch the ground coming out from your flip, this is the moment when his powerfist or chainsword digs into your back.
It is thrilling to throw in some mobility in combat, but remaining stationary for most of the time is no harm either, I found a bit of combination of both to be good, as trying to be spontaneous and flexible in your movement combat variants will never grant your enemy of knowledge of patterns on what you will do next

3A. LONE WOLF
Sometimes you feel this urge to go on a point where no team mates are around, stirr things a little bit on a battlefield. Either draw attention of more than one guy to a point on the back so that maybe front lines will become a little lighter for your kin, or maybe you will inspire some Dire Avenger to follow you in your steps to a clear point and gain advantage that way.
Some people say going lone wolf in a game like this is stupid, but I think when done right it is a very good distraction in a game where every number counts.
By going alone as any class (Warlock in this example) you have a chance of a potential ambush, killing more than 1 enemy if circumstance allows it, and sometimes even drawing attention of 5 space marines to a point that isn't about to be capped, so the diversion is an excellent way to get some numbers off a location upon which main attack is being focused.

So, as a lone wolf Warlock you want to try and get as close to the point unnoticed. I know they dont have cloak like their scorpion kin, but you can still behave in a pretty sneaky manner and manage to evade enemies.
Running for cover and sneaking on crouch is a great way to lurk around enemies that are usually few and plan your initial attack.
Heavy attack to the back is always your goal while sneaking but as we know, good players almost always move around and sometimes when you sneak up on them they either run away, or start running towards you! If you get noticed, a heavy attack is veeeery risky. Most of the times it ends up in your death, no matter if its tactical or ground assault.
If you get spotted it all depends on a distance - if you in around 10 meters of an enemy you can still hope for 2 meter run and attack from run, or doing a front flip and then attacking. If the distance is more than 10-12 meters however and you know you just got spotted, I suggest run where you came from. Back or side flip and retreat! Then you either try to lose them (which rarely happens, enemies are very persistent!), or you run around some corner/s, you ledge onto one, and you strike from shadows upon running towards you enemy and engage!
This tactic is good since usually it is just one enemy that spots you and even though he might point you out, hes the one making a run for you and before his team mates relise whats going on you're already fighting.
Being sneaky, swinging your witchblade and healing up when being a good Warlock can often create a good distraction on the field, and in some cases, if you're really good (or enemy team is shit, I don't know which one it is), you can sneak clear point out of 3-5 unsuspecting marines and signal your team that you need Dire Avenger on that point.
This is a 50/50 situation since sometimes enemy thinks that you're just messing around, lone wolfing as Warlock, not being able to cap, so they don't bother respawning on this point and go on to the front lines, but a lot of times they know that 1 eldar means there might be more very soon, or they simply hold a good grudge on you for that flawless melee victory that you ended in execution and they'll keep respawning until they get you back for it



4. Warlock on whole
As in any faction, a healer/caster class is essential to any deployment. Healing and buffing your team is your main role and once there is a good balance and capacity for more of Warlocks they become an incredible force of healing chains and nicely buffed melee warriors who excel right next to their scorpion kin.
Being healer/caster you perform at the back as well as the front of your team, you have to consider multi tasking with your fellow Warlocks and make good decision who and when should switch from Defensive Warlock to Offensive Warlock. Who will be the healer and if we have such commodity, who will go up front to slay some!
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In the future I'd imagine they will have a second offensive spell such as eldrich bolt.
I don't know if developers are planning on it but if eldrich bolt would make it into the game it could be a great tool in a long and short distance.
I'd imagine it would do some damage (with possible miscasts = 0 or very reduced dmg), as well as stun enemy for brief moment (debatable, 1-3 seconds, dependant on distance?)
I would see that in short distance it would be very short in stun but with more chance of higher dmg and in long distance upon successfull strike + aim it would do something like 3 seconds of stun and much lower dmg.

Anyway, these are my thoughts and a little overview of Warlock class based upon about 50-60 hours of playing as Warlock in Alpha stage of the game.
I really appreciate if you got to this point and if you did, please, let this thred be an informative and opinion exchanging place for Eldar Warlocks!
I would like to hear from other experienced Warlocks and Eldar if they agree or disagree with my points and would gladly read your contributions and observations.

Great thread Oblit, and I'm hoping a lot of successful Locks pop in to share.

Warlock is my main as well and a great character.

I was hoping they would have put in Destructor so I could burn them, burn them all.