Simply an awesome game.
The races, which is scattered full of shortcuts and ramps, will constantly unlock something new for your car. The career mode is very long but always fun.
Also, there's the arcade mode. It's a little disappointing, because you cannot view images of the cars when you are trying to pick one out to race. However, I am not going to lower my "Tilt" score just because of this measly little thing.
There are some problems with this game, though. First off, there is some MAJOR slowdown in some parts of the game. Also, the cars could handle a little better. You'll often find yourself smashing your car into the wall next to you, rather than drifting around the corner.
But these are fairly minor problems. I would also like to point out that I disagree with Alex's comments on Arcade mode. I do not think that arcade mode is lackluster in any way. Games like capture the flag or paint prove to be very entertaining.
If I were you, I'd sell Need for Speed Underground right know. Because once you play Midnight Club 3, it will be all you need.