I've been playing this a little bit over the past week and I have to say it's pretty thoroughly enjoyable. It's got a good mix of retro and new, and blends playstyles of the 2D and 3D Mario games seamlessly.
It does annoy me when there's parts that they've made just to show off the 3D effects of the 3DS though. I am unable to see 3D, so these parts are unnecessarily hard for me, and it's unpleasant that they've made certain parts of their game fairly inaccessable to people like me.
But other than that, I've been having fun. The game was a touch easy for most of it, but 8-8 tested my skills well enough. I defeated Bowser, rescued Peach, the credits rolled... and then it gave me a whole new set of levels. On the one hand, more game! Yeah! On the other hand, that's a rotten sneaky trick. Boo! But 5/5 stars, and I reckon I'll continue to play it on the bus.