Super Paper Mario is a combination between the RPG and side-scrolling platformer genres. You collect points as you kill enemies, and those points count toward increasing your level. As your level increases, you increase either your HP or your attack power (it alternates, you don't get to choose which one).
As with most games of this type, there are a lot of story-based cut scenes which do have an entertaining story to them, but sometimes get long and tedious, especially at the very beginning, where there's a lot of setup.
You start out the game as Mario, after seeing Luigi, Peach and Bowser all sucked into some kind of dimensional vortex. You set off to find and rescue Peach, and recover 8 Pure Hearts to save the world. Along the way you also have a Pixl friend accompanying you to help you along. You find additional Pixls with different abilities (like creating bombs or giving you a super-stomp ability for flattening large objects). As you progress through the game, you find the other characters and add them to your party, and you'll use their unique abilities in different ways. Mario has the ability to do the dimension flip I'll talk about in a second, Peach can use her umbrella to glide long distances or hide from damage, Bowser can wipe out enemies with fire breath and does double stomping damage, and Luigi has a super-high spring jump. There are also at least a dozen or so Pixls with varying abilities that need to be used in different ways.
The game itself is primarily the usual Super Mario action... you run to the right and jump over obstacles and on enemies. One cool thing is that you can shake the remote up and down after jumping on an enemy to get a bonus score. For enemies that take more than one hit, you can do this repeatedly for increasing bonus points. Sometimes you reach obstacles that you can't jump over. This is where the really interesting part of the game comes in.
Early in the game, you learn the ability of "flipping" which is triggered by pressing the "A" button and turns the game world 90 degrees so you can see a different perspective. For instance, if you are going along and run into a really tall pipe that you can't jump over, flipping may show you a path as simple as just walking around it. Flipping also sometimes reveals hidden treasures or paths.
The game overall provides enough challenge, leading you to switch characters and Pixls frequently to solve puzzles, but without getting tedious and repetitive. The space levels were particularly entertaining.
I expect a casual player can finish the game in approximately 30 hours, and it's fun for adults as well as younger kids. It doesn't rely as much on fine motor control skills and precise jumping (though those certainly can help in some situations), but does focus more on making you think how best to use your characters and Pixls to beat certain obstacles or bosses.
I would really only ding this game on replayability... it doesn't strike me as a game I'm going to be likely to pick up and play again once I've finished it. Not because it's a bad game, but because it just won't be different, so what would be the point?