I've played Mario Kart Wii a few times and have to say its not that big of a deal. Mario Kart Wii is not that much of an improvement over Mario Kart Double Dash. Aside from its new characters, Karts and bikes, the graphics aren't much of an improvement. The Wii wheel makes everything difficult. I feel more comfortable playing Mario Kart Double Dash over Mario Kart Wii because i feel more at home with the joy-stick controls. It would be nice if they added some new items. I know they have new items like the Super Mushroom, from The New Super Mario Brothers. I usually play Mario Kart DS and find it, not as good as the Wii, but i prefer it because it has better steering and good items.
Mario Kart Wii has some good new courses. Like "Wario's Gold Mine" Its good because it has interactive features like the gold mine karts. It also move very fast when you are going down a hill or around a turn. Its very hard to make turns because of the controls. If you want to play on some of the really nice courses, you would have to master steering and power sliding.
The battle mode is an improvement from the Mario Kart DS battle mode, it has some great new courses and some older ones too. When you are in battle mode, it doesn't matter who you are playing as because strength, doesn't matter unless you are playing on a course where you can fall off the edge.
I am yet to play the game more and master the steering and the power slides, but when i do that, i think i will enjoy this game very much.