Dark Heresy, check it out. My friends and I have been playing a game of Rifts and one night after a gaming session we went out to eat and the conversation led to 40k and roleplaying it. I mentioned I played DH a few years ago and it was actually pretty fun. It's a brutal RPG and you really have to think about your actions because you can easily be destroyed.
I went to the comic shop last night and picked up the Game Master's Kit for running the game. The quick reference shield is a pretty handy help. When I got back I started to make up the movers and shakers in the world and how the players will interact. So far looks like we'll have between 5 to 8 people playing, a pretty large group for DH. If they stick with it I think they will be facing some serious foes, Chaos Marines and some Daemons .. haha... Whoots. ..
After each game I hope to do a conclusion write up and I'll probably post them up ..