They all handle a little differently, and their minigames and challenges help keep the game from getting monotonous. Some of these activities - like hunting down discarded tires as tire-store owner Luigi, or nailing speeders as Sheriff - are actually a lot of fun, while others (tipping over cow-like tractors as Mater, say) are just mildly enjoyable.
The only really weak points of the single-player game are the repetitive, 12-lap Piston Cup races, but those are rare and usually over pretty quickly. The two-player modes, on the other hand, are disappointingly short and simple. A perfect example is "Sheriff's Hot Pursuit," which dumbs down a potentially interesting police chase by forcing players to weave through traffic on an endless straightaway instead of playing cat-and-mouse through town.