While not as strong as a pure platformer or pure RPG, Super Paper Mario is an enjoyable game nonetheless.
The story is one of the game's biggest strengths. Unlike the standard "Bowser kidnaps Peach" stuff, a new villain by the name of Count Bleck is introduced. Following a prophecy from the Dark Prognosticus, he forces Peach and Bowser into marriage, thus creating the Chaos Heart. This Chaos Heart creates a black hole that will slowly consume all worlds. It's up to Mario to gather the eight pure hearts and use their power to defeat Count Bleck in order to make The Void disappear. Despite being on the predictable side, the story is pretty interesting. It's filled with clever writing, memorable subplots, characters filled with personalities (and some background stories), and a few plot twists. Examples of interesting characters are the crazy Dimentio, bumbling Bowser, bratty Mimi, and the enigmatic Mr. L. Wait a minute..."Enigmatic Mr. L"? Why does that name sound so familiar...
But this writing is a double-edged sword. Have you ever heard of the expression "overdo a good thing"? Well, Super Paper Mario is an example of this saying. Sometimes, there is too much text, which interrupts the flow of the game. You could just scroll through it quickly, but you would be chancing missing something important. I personally didn't get annoyed of the heavy dialogue often, but it is worth mentioning.
The Wii remote held sideways is used to handle all of the controls. It uses some of the motion controls, but doesn't abuse them. You can point at the screen and click on something to learn more about it. Some items require shaking/tilting the remote, and shaking the remote right after landing on an enemy earns points. So, the controls add variety, but they aren't an overbearing gimmick.
The gameplay is innovative. Previously, all of Mario's adventures were either all 2D or all 3D. Here, 2D is the default dimension, but Mario can switch to 3D at any time by pressing A. You can stay in the third dimension for only a little bit at a time, or Mario takes damage. This flip to 3D is one of the gameplay's high points. It is used to solve puzzles in new ways. For example, what looks like a huge gap in 2D gains a thin bridge in 3D. You'd be surprised just how much this flip enhances the game.
Just like the 2D/3D blend, Super Paper Mario is a mixture of a role-playing game and platformer. On one hand, all enemies and Mario himself have HP. After gaining enough points (by defeating enemies and other means) Mario levels up (his HP or his attack power increase). There are plenty of items to collect (from food to weapons), and you unlock more characters as the game progresses. Each character has unique abilities (Mario can flip, Peach can glide using her parasol, Bowser can breath fire, etc.). In addition to multiple characters, there are also these digital creatures called pixels to collect, which act like tools. One is a bomb that can blow up cracked objects, another makes Mario one-dimensional, etc. On the other hand, combat is done in real time (not a turn based battle system), like that of a platformer. The end result it like that one of a classic Zelda game: there are a lot of puzzles to solve (which need careful thinking) and some faster-paced action which complements it.
The biggest knock against the game is that it simply isn't as strong as a pure platformer or pure RPG. If you are an RPG fan, then you'll naturally want things like a turn-based battle system and some badges to (un)equip. Both are absent. Likewise, platformer fans may have wanted some more action rather than puzzle solving. Another notable flaw is the pacing, which is inconsistent. For example, I can recall at least two moments where you have to hold down in a direction for a few minutes, which would have been better either shortened or removed.
As you might expect from a Mario title, the game is fairly easy if you just want to complete the main quest. Finding food and coins (to buy food) is easy, so it is likely you will run into few, if any, gameovers in defeating Count Bleck. Likewise, there aren't many difficult puzzles. Fortunately, Nintendo didn't forget about hardcore gamers. Some optional challenges (see two paragraphs down) will put anyone's skills to the test, making Super Paper Mario a game for gamers of all ages.
The graphics of Super Paper Mario can best be described as "nice". They are colorful with fluid animation, but remember that SPM was originally a gamecube title. With that system's horsepower, the more powerful graphics seen in other Wii titles just aren't possible. Likewise, the sound is "good". The music/sound effects are fitting for each location/action, but there are only a few memorable tunes.
Super Paper Mario runs in at about 15-20 hours for the main quest alone, and the side quests expand this even further. Granted, some side quests (namely recipes and common catch cards) are throwaways, but a good deal of these side quests are interesting enough to experience. The Pits of 100 Trials along with a another challenge (that I shouldn't reveal) add another 25-30 hours of gameplay, making Super Paper Mario a game that will be in your Wii for a while.
CLOSING COMMENTS: Super Paper Mario is a fun, innovative title that is a worthy addition to the Mario series. The writing, while occasionally overused, is funny, and the quest is lengthy with plenty of replay value. The only real problem is that you need to appreciate both platformers and RPGs to enjoy it. If you don't like one, you probably should approach SPM with caution, even if you adore the other genre. Otherwise, I can recommend Super Paper Mario to any Wii owner.