All I can say is fantastic.
The single player career is a good length, 28 races with some additional challenges thrown in. The multi player is great fun, especially with friends. You can also take local multiplayer (splitscreen) online.
Creating tracks, karts and mods is simple to use but will take some time to master, there are already some fantastic creations out there that can be downloaded to give you an idea of what can really be done.
The only draw back right now is the rather long loading times. This is only a temporary issue though as the developers have already confirmed that a patch is on the way to improve this signifcantly. Don't let the load times (or any uninformed whingers) put you off, they will be fixed.
A definate buy. A must have game for your collection.