Simply fun as hell.
Pros:
Highly nostalgic
Gameplay is highly fun and addictive
Replay value is high, this is due to coin collecting and a secret world.
New power ups are fun and makes sense with the games mechanics.
Multiplayer is a absolute chaotic blast and should be experienced.
and last, my princess was NOT in another castle. :)
Theres not really any cons for me except two things I wish they added which could of made this games replay value impossible to die down.
1) A level recorder type system. Even though I love watching pros being better than me in the super skill videos, I wish they would of added something to make gamers even more motivated to replay the game and make their own skill vids.
2) a level creator. which is totally not classic-like as Mario, but I think it'd be fun to make your own mushroom kingdom level and share it among wii users on the net.