I rented it on the w/e... I played it solid over the first day and finished it, got all the achievements for that difficulty and started on the next lot... It didn't impress me when I first got playing it... but once I got through to the mission called "all ghillied up" it completely changed my entire opinion of the game. Its a sniper mission where you are in a ghillie suit with another sniper and you are on an assassination mission, only problem is there is quite a sizable force between you and your target. So you have to evade patrols and helo's. Without giving anything away it was quite enjoyable. The game just completely changed after that (well seemed too) I'm not sure if it was because I was playing differently or what but it really did.
However that said, I'm glad I rented it because I don't rate it as having alot of replay value. Its pretty linear although they tried to cover it up.
But all in all, if you've got a 360 and want some good w/e fun go rent CoD 4