The game's on the demo unit at the store I work at, and we have Wii tethers on the nunchuk and wiimote so people can actually play it. I played some while I was off the clock, and watched others play it. Gotta say, this game looks amazing.
First of all, the soundtrack. Fully orchestrated, and if it's synth, they must be using some of the stuff ZREO is using or something. Beautiful soundtrack. Lots of trips back to old NES songs like the Airship theme from Mario 3.
Graphics are frickin' sweet. They easily rival Corruption. Of course the colors are the bright, classic bold colors of Mario, but there's no other way to describe some of the major cutscenes except "Awesome". Gameplay graphics are pretty friggin sweet, too.
Won't say much bout the story other than Bowser's gotten creative at how he's kidnapping Peach.
Gameplay's pretty nice, too. Easy controls, enough so that any customer coming in has been able to pick it up and play it with little problems at all. The moveset is similar to Mario 64 and then some. There's spin attacks, you have a star gun, walk on walls, etc.
All in all, I can't say I'm a huge Mario fan. But I gotta get this game.