A good port from the xbox
If you are not able to play it on xbox360 for whatever reason, this one is a good alternative. I am not going to say more about its features - you can find reviews of them on the xbox game page - good storyline, serious but believable characters with superb voice acting, good looking graphics and standard run-of-mill sound (not necessarily a bad thing). And the icing on the cake - a huge selection of different human and alien weaponry as well as the ability to play as a human and his alien arch-nemesis (no spoiler here :) ).
The only problem I have with this is the controls.. even at the highest sensitivity, mouse turning is wonky at best. It takes me several 'mouse drags' on a standard mouse pad to do a 180 degree horizontal turn. Annoying but you get used to it after a while - this is the only reason I've taken 0.5 out of its score. Play this game - you won't be disappointed.