Comparing it to the Arcade version, it's got some awesome extra features.
We also have Crazy Box, which is actually very entertaining and challenging. I used to spend hours with my friends trying to best each others top score, often playing Crazy Box longer than the actual game.
I played Crazy Taxi on the Dreamcast exactly once (and I'm not entirely sure it was the same incarnation), and I really didn't see much difference between the DC and GC versions.
My only negative is the lack of a multiplayer mode. Though it was fun to take turns, it would have been nice to see at least a turn based score challenge. I'm sure a split-screen mode would have worked, with both players competing for the best passengers and scores. Maybe we'll see it eventually, but for now I guess passing off the controller is okay.