At the very least, it's on par with Kuribo's Shoe and the Tanooki Suit.
So, what does this power-up--which will debut in Super Mario Galaxy 2--do? No one outside of Nintendo knows for sure, but it seems likely it'll let players create clouds that can be used as platforms.
I'm tellin' ya, May 23rd can't come quickly enough.
Buy: Super Mario Galaxy 2
See also: 'I'm guessing this was a precursor to the propeller suit?'