Way too much level recycling and the alien and pred campaign were far too short. However, despite this, they were /fun/. In both cases, the moment you enter an area, the first thought in your mind is 'now, how am I going to kill everything here without being seen?' In some cases, it's tricky, but damned if it isn't fun.
As for the Marine...heh heh, you're going to need balls of steel. On easy and normal it's too simple - it got to the point where I could BEAT THE PRED BOSS IN MELEE. Hard is where it is at. When you go to hard difficulty, that's when you step into the armour and really wish you hadn't.