I've only been able to play 2 players so far, due to a lack of wii remotes. I have to agree, it's extremely fun. I can only imagine it's a lot more fun with more than 2 players.

Playing single player Mario will never be the same again. I just find it a bit less enjoyable to play the game alone now.

The worst thing is probably the slight pause when someone dies, though. I hate that, and it's messed me up a few times. Also, having to wait for the other player unless if you want to get them killed. The screen only stretches so far.