We just got through a 3 player of New Super Mario Bros Wi. For those still asking how the game is, it's kind of a cross between Mario 3 and New Super Mario Bros on the DS.
Multiplayer can be helpful sometimes but mostly a hinderance, as jumps are interrupted a lot of the time. My advice is, take it with good humour. There's bound to be some tests of friendship on this one...
There is an option to get into a bubble and let another player handle the level until they hit your bubble. Only problem with that is, a lot of mistakes are made when everyone gets in a bubble (level resets), especially with the bubble button being "A" (situated very close to the directional pad, it's easy to hit accidentally).
All up, despite the frustrations, it was still an incredibly fun experience and a lot of the time we could not stop laughing at some of the actions other players caused against us.
