It's like a road trip. The first few hours are awesome, but you'll be snoozing by the end.
The star of the show this time around is the ability to flip from 2D to 3D by tapping the A button. This enables all sorts of clever new ideas for level design, and the first few chapters really show this off well. As an example, you may come to a seemingly impassable cliff, but upon flipping to 3D you'll see that there's a large hole allowing you to walk right through. Items, enemies, and pipes lie hidden behind hills and rocks and can only be found in 3D.
This game is essentially a platform jumper with RPG elements, and NOT the other way around. Do you know why those old Mario games are superb? They were fairly short, the levels were creative, and the situations you were put in never felt repetitive all the way through the game. Super Paper Mario is basically another of these platform games, but stretched to around 12 - 15 hours. 15 hours of similar platform jumping, enemy stomping, and flipping tricks gets tiring. Which brings me to level design. As I said, the first chapters are all very creative. You'll do different things and explore new areas. As you get deeper in though you'll come across more and more puzzles. It chops up the fluid platforming which is so inherently fun. As an example, in one of the desert levels you'll come across three plain blocks at a dead end. To pass, you'll have to get the secret pass code, which is not at all obvious to find, and then input it on the blocks. It's frustrating, pointless, and serves no purpose other than to slow down gameplay.
Another later level entails running to the right for about a minute and a half. That's it. No enemies. No stage hazards. Nothing. Just run for a minute, and then you finish. Fun?
To try to spice things up, you get four characters total. Really though, you'll use Mario the whole time since he's the most agile of them all. Bowser is incredibly bulky and slow; Peach is better and can float, but isn't all that satisfying to run around with; and Luigi doesn't control tight enough for tougher spots. Even using Mario, you'll notice that the controls aren't as tight as they should be. I fell in many pits when Mario refused to jump. I also got hit a lot trying to move in close to hit enemies with my hammer. It's almost like you have to wait for some animations to finish before another can begin.
The story in this game is fairly weak as well. The characters are entertaining, but I never felt connected to the world or the characters. When disastrous events occurred, my reaction wasn't "Oh my gosh," it was "*sigh* ok, what's next?" The final chapter pulls out some classic RPG story tricks, and they're very shallow and fairly ineffective.
So that's a lot of bad so far. Why should you play this game? Because the first half really is an awesome experience. The art style of the characters and enemies is charming, and the dialogue is mostly entertaining. Also, exploring the town of Flipside (and the other town you discover along the way) is fairly rewarding as well. There are a few extra things to do if you're really devoted (or just really REALLY bored) like collecting cards and recipes. There are also two extra dungeons to run through, but the reward for doing so isn't worth it. Even if the puzzles are frustrating and boring, the platforming is classic goodness with a 3D twist.