Best game on the GBA, period.
The sound is actually really good, for a GBA game. Believe it or not, the GBA is actually capable of voice acting! I kid you not! There are full motion cut scenes, but they rarely have both full motion cut scenes and voice acting at the same time, but that's understandable, given the limitations of the system.
The music is really awesome. The GBA didn't have one of the best sound processors in the world, but I couldn't care less. Give me a catchy tune in the background, and some sound effects so that I'll know what just happened, and I'll be content.
The graphics are also pretty cool. For a GBA game, this is top of the line. If you play the game on a big screen, like on an emulator, it will look pixelated, but if you play the game on the GBA, or on an emulator that is the size of a GBA screen, the game will look crisp and sharp. The levels are brightly colored, and you'll never wonder if something is a friend, or foe, because everything is clearly identified in the game.
Speaking of the levels, they are full of challenging obstacles and tough enemies. There are spikes, lava, moving parts, and monsters galore, and touching any of them will kill you instantly. You have no health meter, no super mushrooms so that you can take a hit and still keep going; you don't even have any invincibility stars. This adds a whole new degree of difficulty. You can only rely on yourself.
Sound punishing? It isn't. The game almost always plays fair. Most of the time, if you don't manage to beat the level, it's because you messed up. You mistimed your jump, or you didn't duck at the right moment. The game even gives you a video giving you a hint on how to pass the next level.
Of course, I say "most of the time" and the game "almost" always plays fair, because there is one level in the game that does not allow for a single margin of error, and that is the first half of level 5-3. That level almost made me slam the controller down in frustration. It's the hardest level in the game. I had to look online for the solution to this level, and, when I figured out the solution, I still had trouble. Besides, the solution is so unobvious, I don't see how anybody can figure it out on their own.
This level is what made me give this game a 9.5, instead of a perfect 10. One of the biggest complaints about Superman 64 is that it had extremely difficult mission objectives that allowed for no margin of error, so, if they can whine and complain about a game being bad for that reason, then I can whine and complain about a game being imperfect for this reason. It's only fair.
But, the rest of the levels are top notch. Challenging puzzles (but not "How the hell is anyone supposed to beat this" challenging) and fun obstacles litter every other level in the game. There are 65 levels in the game. Eight worlds, with eight levels in each of them. The first six levels in each world involve getting a key in the first half of the level, and rescuing a Mario toy in the second half (that's the game's story; Donkey Kong has stolen all the mini Mario toys, and the real Mario is giving chase, to get the toys back). In the seventh level in each world, Mario must take the toys and put them back in the toybox for safekeeping. The eighth level in each world is the boss fight against Donkey Kong. The 65th level in the game is a second boss fight against DK, and it's really hard. Not as hard as the first half of level 5-3, but it's still pretty hard.
The game is probably the best platform game on the GBA, and you'd be wise to pick it up.