One of the best games available for the GBA. It's old school Metroid with better graphics and better game play. You'll be hard pressed to find a better platform game anywhere.
There's lots of different weapons to collect and use. There's also several suits you can get like ones that let you walk through water and over fire.
The controls are near perfect. The puzzles are at times annoying, in that you'll search a room for hours trying to find a way to the next level, only to find out the only brick you neglected to bomb was the one that lets you get out of the room.
Once you finish the game, you can connect your GBA to your GameCube (if you have Metroid Prime) and play the original NES Metroid!