I just played for about 15 minutes at Gamestop.
My favorite characters to play with are, in order, Fox, Pit and Diddy Kong. Fox is nice and fast, which I like. Pit is also fast, and has uber jump and recovery, especially his up-B. Diddy has a nice feel of power and speed (not as slow as DK or Bowser but definitely not as fast as Fox or Pit). I didn't try many other characters, but I imagine if I get the chance to play again, Sonic and Wolf would be favorites of mine as well, since I like the speedy characters.
My favorite stage so far is the Warioware one. At first I just thought it looked cool, but then I got thrown into a Microgame while I was fighting Wario, which I thought was really innovative and fun.
I like the setup for the Subspace Emissary. It's very adventure-mode-y. I played it until I got to the DK-Diddy level, then stopped to try Fox and some other characters.