Great single player, great levels, but ultimately, a modest improvement at best for the series.
Single Player:
Interesting design, two-player karts are neat, mainly when deciding what weight and 'specials' you want with your team.
Fun levels, some a tip-of-the-hat to N64's MK.
Good Ai, if you fall behind there's better opportunities to catch up, while if you're in first, it's all banana peels and green shells (which means 'weaksauce')
Multiplayer:
Boooo!
The vs. levels are boring, flat and often end far too quickly, the addition of a few new game-types was a meager breath of life in a sinking ship.
The two-on-one kart option might as well be taken off, becuase it's harder to control, and one person is almost always doing nothing.
Overall:
If you like Mario, especially Mario-Kart, of course you should get it, but lets get real, next game for a new system should offer more, and a kooky design isn't the answer.