I played the PC version and loved it,never finished the game because it gets really hard later on and i just used God mode.
Co-op sounded like fun as well.I have to say that Vicarious Vision made an outstanding job on this...it looks amazing and runs at a steady 30fps,it looks better then Riddick on the XBox,you can save anywhere,there's even a quicksave button,the controller works really well...there's a little more loading then in the PC version,but it's not that bad and the loading screen is about 10 seconds.
I got to play some Co-op and it's a great ride...it's most of the single player game but with someone along...they added a few darker area's where someone absolutly needs to use the flashlight so it's very tense in some area.The 4 player Deathmatches and the like are very fast though but it's still fun i thought.
I'll be online with this most of the rest of the week so if you feel like playing online,the co-op especially,send a message.This game is at least worth a rental just to experience this mode alone.