I have gotta say that Test Drive Overdrive is the best game in the whole series.
Graphically, its actually very good. Sure, not up to par with GT3, but still very good. Excellent track design too.
Gameplay is classic TD stuff, and there is nothing wrong with that. It has stacks of tracks and stacks of cars, and lotsa different modes including drag races, cop chases and the usual time attack sorta races.
Very good game.