Not nearly as bad as many people say, but not as good either. I personally loved it though...
The story in this one is quite simple, but also interesting: the evil Count Bleck in an evil... count who wants to destroy all worlds using the dark Chaos Emerald...Dah... I mean the Chaos Heart!. It all starts like a tipic Paper Mario day, but there's a lot of character developement as the game progresses, especially in Bleck himself.
"Super Paper Mario", unlike its predecessors, is actually an old-school platform with RPG elements that appeared in previous games of the series. Exp points, HP, and other "RPG-things" are completely usual in this game. "Switch" is, however, the real "name" of this chapter. Most of the game's puzzles must be solved by literally switching from 2D to 3D and also allows the player to easily manuever around obstacles impassable in the 2D (such as pipes). For this reason, it has been often described as the first "5D" platformer in some reviews (not including this one)..
During the game you will be able to play with other "classic" characters, including Bowser and Luigi. Each character have one special ability, but Mario is the only one who is able to switch between 2D and 3D.
If you're a Mario fan, then you'll probably going to love this game, just don't expect "the best" Mario game ever.