Metroid Fusion is a game in the top echelon of games for GameBoy Advance, and for a good reason. While this may not sound that hard, but if you look at the competition, it's not that easy.
The graphics of Metroid Fusion are top notch. I don't see 2D getting any better on a system like the GBA. The colors are great, the enviornments are detailed as are the characters. Everything in the game looks great. Be warned, however, once you go from Metroid Prime it may take a while to get used to. This may sound stupid since you would think that's the case after playing any 3D game, but just wait until it happens.
The sound of Metroid Fusion is also in the top of its class. From the monster sounds to the music, it's all great. The bosses all sound great, while some may be a little bit cheesy, they are still amazing from the GameBoy Advance. The music is also something that you would not expect to be on a GameBoy Advance, but it is nothing to compare to next-gen console games.
Finally, the gameplay is what will keep you involved in Metroid Fusion. While it is somewhat short (not as short as you would think a 5 hour long game would be, however), it's great fun. This game is the epitome of a great platformer. You won't find better anywhere else, if you want my opinion on the matter. The only real drawback, like I said, however, is the length. It's not going to be something you'll want to play over and over again. One of the reasons why I say this is because you can't start over with weapons and suits that you have already aquired like in previous Metroid games.