Update: I was a bit harsh before, I still think it's got to make a few improvements.
Starting with a good point, the graphics are very good. Not super smash bros. equivalent but Mario and his chums still look pretty good. And bringing all the old stages together was quite cool.
The stunt jumps and bikes don't make a massive impact, even though they are new. The new karts and bikes look great, but all the useless info to the side just make it that little bit more upsetting. "Wow!" I said, when I had done my first stunt jump (on a bike), a few minutes later the hype was gone.
The online play (from watching, not experiencing) is really competitive, you can't just think you're going to win as soon as you buy it, nooooo way!!! There is tons of stuff to do online, including breaking records. If you download the Mario kart wii channel there is an option to break ghost records, which is really fun! But also slightly annoying.
Overall, this is not a bad racing game. Classic, funny and competitive, but that doesn't change the fact that it is very shrouded by the racing and winning elements that you sometimes forget yourself and go "Oh ****" when you come second place by less than a couple of milliseconds. If you are a dedicated Mario fan, buy it as you will, but if you're into it for the hype, then buy it at your own risk.