I've logged several hours on this game now, so I thought I'd dash off a few quick thoughts for anyone who was considering buying this game.
First of all, it's hilarious. When I read about how it was supposed to be nonstop humor, I was half-expecting weak jokes aimed at little kids, but I've nearly hit the floor laughing on more than one occasion. Not only blatant jokes but a lot of little subtle things going on apart from the main action. And refences to other Mario games, so so many references. You'll have to be an old school Nintendo fan to catch everything they throw at you, including knods to Dr. Mario and 80s Nintendo commercials.
The action is good, as well. It takes a little bit to get used to the controls, but once you do it's all good. Overall, it's a great game and Nintendo has lived up to the high standards of several of their other games.
Oh, and if you've ever wanted to see Mario running around in his underwear... well, you'll get it, but you should still seek professional help.
I tried to give it a chance, I really did. All in all, it's Driver 2 all over again. The few things that they added really didn't do anything for me, and it feels, to me, like a game that wasn't finished yet. I especially don't like the AI.