I picked up Super Paper Mario for the Wii today. It was only released on Tuesday, so this review is a bit more applicable.
The Paper Mario series is a set of very fun, very family-friendly games that Nintendo released. In these games, Mario is 2D, and that aspect comes into play when Mario must roll up into a tube to slide into small spaces, fold himself into a paper-airplane to navigate chasms, and so forth. Lots of fun, and the graphics reflect the "paper" theme, which makes it pretty cute without being overbearing.
I never played the first one, released on the N64. The Gamecube release, The Thousand Year Door, was an action/RPG of great fun (if you haven't played it, go get it!)
This installment in the series leaves behind the best of the RPG elements, which is a real shame. The game is 95% platformer now, with about 5% RPG still involved (finding items, growing stronger by gaining experience). It's still a fun platformer, but I miss the turn-based battles where the timing of button-pressing affected damage (etc.)
Well, this shouldn't be a review of the game I wanted. The game plays like the old NES Mario games, but with a twist; when you press the right button, the world swings around and you now have a 3D view. Can't get past a group of Thwomps or a tall pipe? Rotate the world and just walk around. It's a pretty clever idea, although it does get a little disorienting to do it so often. The game also puts a timer on you, where staying in the "rotated" mode for too long results in damage to your character. This seems like an unnecessary feature, given how integral the swapping of views is to play the game.
I only played through the first level (~2hrs of play), so these are only initial thoughts. The spirit of the game is true to the series, and it's been fun. Pick it up if you can find a copy (but I'd recommend the last installment over this one).