I recently replayed Metroid Fusion for the Game Boy Advance. In terms of graphics the game still holds up nicely, it doesn't look quite as good as say a Castlevania on the NintendoDS, but as far as the GBA is concerned it is definitely one of the better looking games. Sound and music are also pretty good. The controls are a bit overloaded due to the lack of buttons on the DS, but work overall quite nicely. The L button lets you aim up or down in a 45 degree angle, holding R lets you shoot missiles or charge missiles and A and B let you shoot and jump. Pressing start will show you a map of the explored area.
Like previous Metroid games this one also provides a chain of of collectable extras that give you new abilities, you get those collectables after basically every boss fight. It is the standard stuff for most part, morph ball, bombs, high jump, screw attack, etc. The one new thing is that you now got an chargeable ice missile instead of a regular ice shoot, but it serves much the same purpose as before.
One nice new core game mechanic is the way how you refresh your energy. Like in previous games enemies that have been killed will leave collectibles that will refresh you missiles or energy, but in Metroid Fusions those collectibles are actually viruses that will, if not collected quickly, respawn into a new enemy. It is also possible to kill two smaller enemies and then have their viruses respawn together as a single bigger enemy. This mechanic gives the game a great sense of speed and urgency as you end up constantly hopping around and collecting things.
Unlike Super Metroid however, this one is a lot more talky. The game is separated into 7 sectors and at the entrance to each sector you will have a short dialog with an AI computer. While every now and then this provides interesting story bits, it sometimes feels a little to much as all you get is another objective to collect this or that item. The story driven nature of this game also adds a bunch of linearity to the game. You always have a next objective to fulfill and there is very little room or reason to go exploring and due to the way many secrets are only accessible with newer power ups that you might not yet have. The one point where you can freely explore is right before the very last boss fight and while it is fun to revisit old locations and explore every spot of the map, it is also rather pointless, as at that point you simply do not need any of the extras any more. Unlike the Metroid Prime series, collectable items will be marked on a map, this makes it much easier to keep track of what got collected.
The one area where the story telling in Metroid Fusion really shines are the encounters with SA-X. SA-X is an fully powered up Samus clone in the Super Metroid suit that you will run across multiple times in the game. It is undefeatable, thus your only job is to survive the encounters. This can be accomplished by either freezing it with ice missiles or hiding. The music, pacing and the fact that they all play out through normal gameplay makes those encounters extremely suspendsful. SA-X and other boss enemies will will also from time to time leave a trail of destruction behind them, thus blocking previously accessible path and forcing you to find another way around.
The difficult of the game is pretty well balanced, providing a decent challenge, without ever getting to hard. The once exception is the Nightmare boss, that one is by far the hardest boss in the game and takes by far the longest to defeat compared to other bosses. The game also has a few situations where it is not all that obvious you have to do. In one instance a locked door will only open up, after you defeated some enemies that only appear after two viruses combined. While having doors open up when all enemies are defeated is not a new mechanic in games in general, it is used only exactly once in the whole game and there really isn't any reason given why that happens. In those cases you are mostly looked up into a rather tiny environment, so there isn't all that much chance that you wander off into a completly wrong direction, but in those few cases where this becomes an issue a few additional hints would have been welcome.
In terms of length the game is not the longest, but fine for a handheld game, an average play through will probably take you around five hours, collecting all the items might almost double.
Overall this is probably my favorite Game Boy Advance game. While the way the story adds linearity and slows things down can be annoying, the story itself is perfectly fine for a handheld game and serves well in giving you points to go to and explanations for new game mechanics. Some non-linearity would however have been welcome instead of giving the player a completely guided experience. There is still a bit of exploration to be done, as your objectives might not give you an exact room to go to or tell you how to get there, but you don't really get to do exploration on a larger scale till the very end. The nice thing about the exploration however is that you will learn a few new moves that you likely never used in the normal game, so while the new energy tanks might be rather useless, you actually do see a few new and interesting things and some items provide a good challenge to figure out.