Just tried this thing and it doesn't seem to do all that much. It has ridiculous recoil for some reason, too, making it very un-friendly to use.
Have you played against it? It does the damage of a Bolter, has the range of a Bolter, damages vehicles and slows to boot. There's no reason to take anything else. I assume it costs more than a Bolter, but that's its only downside.
Out of all the faction, LSM needed new weapons the least. Eldar would have loved a Firepike, or more unique weapons. or hell, even have our current uniques fixed.
It's like playing PvP in an MMO and having a sorcerer spam disruption-based spells at you. Nobody likes playing against things that can slow or disable their character when it is out of their control and unavoidable.
Personally i dont have a problem with base grav weapons.At least not a big problem ....its the fact that they gave LSM 3 uniques on top of that when other factions needed attention ...desperately.
The main issue isn't grav itself, it's that LSM has now gotten 6 (9?) guns whereas no other faction has gotten any. LSM gets 5 free unlocks at level 3 and much cheaper LP costs overall from what I can tell. It's dev favoritism. Grav itself, ignoring snapgrav because I haven't seen anyone use it yet, is weaker than a bolter at ranged and much stronger than a bolter at melee. What I mean by that is that if I'm playing a tac equiv and I start getting grav'd I have time to turn around and win a ranged 1v1. As melee you better hope you have a pistol because you're not catching them unless they mess up. Vehicle wise it's very strong. It does nice damage but not vulcan multimelta damage. Gravcannon is incredibly strong versus these. If you combine vulcan MM with grav it's back to the days of vehicles dying without being able to respond.
Personally, it doesn't really bother me, but that's probably because I don't melee much. My Shoota and Big Shoota can out-DPS it if it's just one guy shooting me with it. One well placed Rokkit does the trick as well.