I've only played Mario Kart Wii, but I've been enjoying it. I've never really been into any of the Kart games since the original SNES version, but MKWii is a lot of fun. Online was a surprisingly fun experience as well. I've only played a few games, but I thought for sure I would be obliterated by Japanese teenagers. Instead, I managed to hold my own.
My only complaint is that the single player can get ridiculous. Even on 50cc, the easiest mode, I am constantly blasted with red and blue shells, lightning bolts, POW blocks, etc. This doesn't usually prevent me from finishing first, it's just annoying. However, as I've played more and mastered the controls (relearning how to drift boost, discovering the boosts gained from performing midair tricks, etc.) I've been enjoying it more and have not been annoyed by the CPU as much.
There are a lot of tracks (although half of them are ported from previous versions), and different game modes.
I haven't logged much time with it, as I just got it a couple weeks ago, but I give it a :thumbsup:
EDIT: Oh, a couple of other complaints (related): No multiplayer Grand Prix mode. What's the deal with that? And I guess it kind of sucks that only one "Mii" can be logged in at a time.