It is Mario Kart, and like any Mario Kart, it is a must buy, but that does not mean the game is flawless.
The online mode plays smoothly. Unlike other Wii games, you connect relatively quickly. It is not difficult to connect to a friend, and get your kart on.
Some of the downfalls of Mario Kart Wii are the new weapons. Any Mario Kart veteran can tell you stories of losing the game at the last second because of red shell, or lightning bolt at the wrong time, but Mario Kart Wii takes this to a whole new level. Much of the Single Player Grand Prix is luck. Do not expect to take first and hold it for any extended period of time. Blue shells constantly appear. Lightning Bolts constantly appear. A new weapon, the POW block, constantly appears. Item barrages are also commonplace. Blue Shell, Shrunk, red shell. The POW block also has an interesting side effect, it takes away any items you are currently holding. Now, on an average size track you experience two of these a lap. This means that any mushroom required shortcuts are very hard to take, for in many instances you will find yourself either having to use your mushroom right away, or risk losing it.
All in all Mario Kart is a very good game. It will provide you with of hours of fun, and is definitely a must own for the Wii. All craziness of the single player aside, five years from now when you are playing this game with some friends, you will be happy you own it.