Mario's the only one who can pull off this feat, but the other three aren't useless by any means. Peach, in a wonderful addition, uses her parasol like a glider (yes, like Super Mario Bros. 2 ); Bowser can breathe fire and deal heavier damage; and Luigi is able to bust out a super jump that rockets him right out of the screen. Certain areas require a lot of time sharing between heroes, but for the most part you'll be Mario, only switching to one of the others when they're needed. The axis flip is crucial to playing the game - breathing fire rarely is.
The various Pixls you acquire also go through periods of use and then total disuse. These little holographic critters are where your power comes from, but only one can be equipped at any time. It would have been nice to have the throwing power of Thoreau and the gliding ability of Carrie at the same time, but you're forced to menu-surf quite a bit. Other powers include shrinking, swinging a hammer, doing the butt-stomp, planting bombs and flipping areas of the screen like a window.
The point to gaining these powers is, naturally, to advance further in the game. And much like a Metroid or Castlevania game, your new powers let you go back and revisit old places for greater access. Thing is, replaying an entire level just to check out one potential lead, then be forced to finish the entire thing is incredibly grating. Why can't you get in, do your thing, pause and hop out of the world?
While the charm does start strong (the first two days of playing in the office drew a fairly consistent crowd, about the same number of people as the GTA IV trailer), everything starts to peter out close to the end. The plot thickens and loses its silliness, the different Pixls get used less and less and the levels start becoming more and more about wandering around looking for crap instead of having fun. So, much like Wind Waker, Metroid Prime 2 and Super Mario Sunshine, it's easy to get 75% through this and call it a day. Call it a GameCube Curse.