Plasma weapons are good! Easily work out the best value weapons using the standard rulebooks.
Plasma pistol is much better than the bolt pistol for only a small amount more. Better ammo roll, and option to crank up the power. Plasmagun is miles better than the heavy stubber [IMO, although some will disagree I think] and it is certainly a good deal on price when you compare it to meltaguns, grenade launchers and flamers, which it competes with in special weapons.
The heavy plasma gun/plasma cannon is comparable to the other roughly 300 cred heavy weapons such as the autocannon and missile launcher using crack missiles. I prefer the autocannon here, but there is nothing wrong with the heavy plasma gun.
We only use the official rules/faq, so disarm is seriously powerful, if you get it. But so far noone has rolled the skill in my group, and even if you did, you need it on a guy who isnt going to die in combat. Sure it could be useful against a heavy, but how many people get heavies in hand to hand? If that happens to me, my whole gang must have been overrun, and I am likely to have long since bottled!