It really is a great game, probably the best MK so far (though it is almost a tie with DD and DS, they are both amazing too)
The bikes are a good addition, though they are not "clearly the faster vehicle". It really depends on the track wether you are faster with a bike or with a kart.
What is, in my eyes, the best improvement in MK Wii besides the fact that there are twelve karts competing is the possibility to unlock things with star ranks and time trials. This gives the game a far longer playtime in single player.