This game is pretty darn easy.  I beat most of the tracks with only a rental.  It's not near as tough as the SNES version.  Heck, the SNES version of Rainbow Road had no walls around the track and the track was thin to boot.  Rainbow Road is a cakewalk on this one.  Wide the whole way with walls all around.  Too darn easy.  However, there is a monster shortcut that's very dangerous to go for if you want to make things interesting.