Borrowed a copy of Need for Speed: Undercover for X360 from a friend and it's more pure fun than any game I've played in a while, including Fallout3, Gears of War 2, etc. I think the last game I enjoyed this much was Call of Duty 4.

I never was that big into the tuning/custom car scene, but it's a lot of fun in this game to tune your car and add car parts to "customize" your car, though there isn't that much variety or options available.

The racing mechanics and types of races are great. It doesn't have realistic driving physics (you can weave through traffic at 180mph), but I've always felt that driving games that are too realistic make it impossible to win, and therefore you stop playing. I've been playing this game almost nonstop for 2 days.