Going into Fusion, I'd played every Metroid game other than Prime Hunters and Pinball, so I knew what to expect and I was expecting to enjoy it quite a bit more than I did.
Let me be frank, Fusion isn't a bad game, but having played every other 2D Metroid, I can say it's my least favorite besides the very original. All the elements of a good Metroid game are there; useful pickups throughout the world, rewards for searching previous areas, a gallery of bosses with their own idiosyncrasies, and there's even a fairly intricate plot that tries to give Samus somewhat of a personality.
The devil, however, is in the details. This is very much a game I felt I would've had a much easier time with had I been using a guide, and despite them being quite a bit more non-linear, I never had that problem with Super Metroid or Zero Mission. This is likely because those games did not love to utilize hidden morph ball tunnels quite as much, nor did they like to hide the correct path a few rooms away from a dead end, as this one does. Other non-intuitive decisions involved: near requiring you to use a power bomb to reveal a structure to aid in defeating a boss and giving you no indication that you have to do so, requiring you to kill yellow pirates to open (with one exception, optional) white hatches and again, giving you no indication of how to do so or even how to make them appear, and completely cutting off any and all exploration or hope of getting remaining pickups after a certain point without giving you any indication that you've reached that point. And then putting save points after said point.
Presentation-wise, the graphics look just as good as Zero Mission, which is to say, very nice by early GBA standards. I've never really held Metroid soundtracks in high esteem (other than the Prime series), and this game is no exception. The music isn't bad, just a little too atmospheric for my taste.
While I don't have a problem with the idea of a linear Metroid game, I'd like one a little less cryptic and a little more forgiving. Metroid 2 and Prime 3 made it work, and I wish this one could have as well. I'd recommend giving other Metroid games a shot before picking up this one. It's still a good game, but it pales in comparison to the rest of the series.