I've used it, and its not terribly overpowering. It has a homing effect, but I could still dodge them if I knew they were coming, and it only has like 10 shots. Besides, you're painting a huge bullseye on your back, and people tend to notice the spartan with a giant plasma launcher, and take him down fast. In the games I played, nobody got more than 3-4 kills with it before dying.
That said, I'd still cut the ammo to 4-5 instead of 10-12. I'm sick of power weapons...