To re-state what a lot of people think about them, I think they are good -- when used as a supplement to the rest of your list. Spamming them isn't such a good idea.
When comparing them to plasma, I see them being better when taken the following 2 ways:
1. Bike Squads
2. Centurions
Personally I don't use bikes, so I haven't tried them, but on paper we can see it will be a powerful platform.
I have used Centurions in a couple games and found them to be pretty damn devastating. The sheer volume of fire from the Grav-Cannons and the re-roll to wound really does it's job both against vehicles and infantry.
I'm glad I bought them