Penny Arcade says to really enjoy MK you need as many GCs, TVs and GBAs you can get your hands on. Not to mention friends to operate them. They did get the full four kart, eight player race thing going but said whilst it was the best multiplayer race they'd ever had they'd probably never bother to pull it all together again.
It seems to me that as MK gets to be less about the actual racing (see SNES vs N64), you're going to need to be getting a pile of mates around to get your two penn'orth. Not necessarily a bad thing, but in deciding which game to get ask yourself this - do you have MK-playing friends you can call on?