You'll need friends to enjoy this game to its fullest!!
While MK offers nice variety of racing tracks and characters, all in the mario universe, you will probably find it a bit boring after a while to compete against the AI of the game. All difficulties are pretty much the same since opponents never really get much better. You will find yourself winning cups without challenge fast. The controls are simple and easy to learn. The sounds are okay and the scoring system is easy to understand.
However, if you've got friends to play with, then it gets a lot more fun. The use of items/traps in races makes it more than just a driving competition. The battle system is also a great feature where you get to fight in original and interesting arenas.
In overall, I'd say this game deserves a good score because of the originality of the tracks, but mostly because of the multiplayer. Had the AI been slightly more challenging, it would have made solo gameplay a lot more fun.