Awesome, awesome game. If you like fps's, then play this game. Immediately.
This game is easily the best fps for the Xbox in terms of single-player story mode. It's highly immersive and engaging. It's fairly challenging on the the highest difficulty. If you are a seasoned gamer, definitely play it on the harder difficulty, it's more enjoyable. If for some reason it's too hard, you can always change difficulties in mid-game. (I never needed too, and I consider myself a seasoned but mediocre fps player)
Why are you still reading this? If you've never played this for the pc, by all means go and pick up a copy, I beat it in about a week, so you should probably just rent it.