Three things are fantastic to Metroid Fusion; Gameplay, gameplay, gameplay.
Wow! You really 'control' Samus through these mazes -- you are doing everything and it is rather simple. The gameplay is far better than any GBA I have played to this day. You really have to experience it.
The graphics are neat-o as well. I especially love how when you kill a monster this floatee pops out of it for you to claim -- if you don't monsters will regenerate.
The puzzles are difficult, but not too difficult (yet). They are fun to spend hours figuring out and pondering. Once you pass one part they keep you concealed so you don't have too many rooms to figure out what to do if you're stuck. (Opposite of Zelda)
The storyline is fun -- makes good sense.
This game is excellent and I recommend it to everyone who owns a GBA.