Yeah, that and Oblivion were the first 2 games I grabbed when I got an Xbox last year. I loved it, but there were a few things that could have been done better: Like the fact the game's ridiculously hard until your in the 40's for level. My advice: either just stick out the first couple of games as long as you can so you'll get more exp at the ending, and worry about doing it well later once you're in the high levels. That or get a car and drive around in the tunnels for hours getting the Zombie Genocider achievement. Boring, but you get an acheivement, a crapton of exp, and (If I remember correctly) the gun from Megaman which lets you kill all the bosses easily. Makes the game kind of cheap,but the boss fights are the worst part anyway.
And I know I've said this before, but get Left 4 Dead. It's sweet.