My big complaint is that it keeps locking you out of areas you've already been to. That just doesn't feel like the Metroid explore-everywhere spirit.
(one particular offender is the upper corner of Sector 3. You get one shot at the hidden power-ups there. You miss them and you can't get back there until the end of the game.
SPOILER SPOILER And the endgame itself... when you can find the tunnels to travel between Sections directly, the next time you visit a Navigation Room, you're told to go to the final boss. But then the front entrances to all the Sectors become locked. They only way back in is that hidden back door to Sector 2, and then you can only get to the other sectors through the hidden warp tunnels. That's just a dick-move.
END SPOILER SPOILER)
Not a bad game on its own, just doesn't feel like it should be a "proper" Metroid because of that.

I just got terribly lost trying to reach Serris too. Don't tell me what to do I want to find out myself :3

Also, is it just me or is wall jumping very hard to do?

@KingMike
For some that's sort of annoying but for me, it's more convenient. I like those passages a lot. The difficulty can be sort of dreadful too.

@Morpheel
Nintendo nerf'd the wall jumping so sequence breaking was much harder to perform. Notice how you can't reach the same wall you jumped off of when done.

Also, a tip for Serris: Do not run around. Stay on one platform crouching and shooting missiles.

Wall-jumping is still easier to do than Super Metroid (though as mentioned, less useful).
Spin-jump towards the wall, then press the opposite direction and jump again when you touch. (I think you just need the timing right, unlike Super Metroid, where I think you needed to be accurate down to certain frames.
Why much of the Super Metroid mod community seems to feel the need to make wall-jumping as essential a skill as walking, I don't know... )

Damnit, every time I see Metroid Fusion, I think of Sega Fusion, and every time I think of Sega Fusion, I think of MEGA FU~SION!

Loving this a lot. spoiler Barely beat that sector 3 part where you have to make it in under 6 minutes with 17 seconds left to spare! * END Spoilers* I was busy looking for upgrades. Lol, that was intense.

just to warn everyone, this referrs to his spoilers section.

I found that once you break back into sector 2 at the end of the game (if you so choose), the locks on all the navigation rooms dissapear, so you can just go to the sections as normal. Although I've found the secret tunnel, I don't have to rely on it.

Oh I just beat that part! That was intense
Now I'm having problems with a certain bug who loves to do huge amounts of damage.

I don't know whether I like Metroid Fusion as much as I love Metroid II. Everyone hates the early Metroids simply because they don't have maps or they aren't in colour or they don't have save points (ala Metroid NES). But I love them, I have three versions of Metroid (GBA Remake that comes with the original, 3DS VC and just recently bought the Original NES version) and I also recently downloaded Metroid II: Return of Samus and as I said before, I love it as much as the first game.
I don't know why I love the 8-bit metroids or NES games in general, but they just have this appeal that modern games today don't. Also being a Metroid fan, i've played all the 2D Metroids, but I loathe the Prime games because i'm not really a First Person shooter/ adventure type of gamer that enjoys those games.
I can't remember the last time I played Fusion, but I still love the game (and own it), after all it was the game that introduced me to the series, before that I only knew Samus from playing loads of Smash Bros. on N64.
I'd disagree with anyone that said Other M was terrible, that game had great hype up until release, doesn't anyone remember that? or have you all just forgotten??
I can't class it as either 2D or 3D because really it's both (minus the scanning from the Prime series). Either way, Other M covers pretty much what left everyone wondering when we all originally played Fusion , like who was Adam Malkovich and where the Nightmare came from.

I FINALLY killed that spider.
Oh I hated that thing, I had to memorize it's attack pattern to kill it in like my 30th try, seriously!

But it was worth it. I ended up killing it with a 90% of my health intact and that feeling of accomplishment games can give you.

I don't understand how this boss causes troubles to some people. You can pretty much stay out of its way by hiding in corner in morph ball and come and shoot when it's mouth is open. It gets a little harder when it starts jumping like crazy but still not too hard. I killed it at first try

Thing is not everyone notices it follows a rather simple pattern at first.

Also, it has a tendency to burn corners and its bouncy nature makes it change its own corner (the one that is not safe because it can reach you if you stay there) every now and then.

And its grab attack did 2 complete tanks of damage to me.
Not to mention that Samus has the strange tendency to fall to the side the spider's most likely to move to and ends up grabbed again.
Most of these tries were combo chains ._.

_{... It may also have something to do with the fact that I have troubles going from morphball to normal and shoot missiles quickly. The SA-X scene after this was a (shorter) nightmare, too.}

The second part of the fight was much easier because all you need to do is stand in a corner and shoot missiles in diagonal and upwards when it is in position.

ok, I try to avoid Other M discussion but wow this is a bad example. Yes, a game that was hyped up but then dissappointed a lot of people isn't a game that is liked by those people. SURPRISE.

Yes people are completely hypocrites for being dissappointed with a game. Apparently.

Oh man, I just beat that boss too! Geez, that was hard! It took me like 5 tries! I was doing pretty good only getting caught in his mouth once, but then he started hoping everywhere. That caught me off guard, and almost killed me. I was so worried! What a satisfying victory.

Yeah, but now I'm stuck with Nightmare... The strange thing is that that battle doesn't make want to really try and kill it, it's quite boring.

Beat that plant thing. I thought that was nightmare and was about to come here to brag it was kind of easy. Decided to stop before I went up against the actual thing, since apparently he's hard.

I liked Nightmare. It's an awesome boss and also the hardest boss in Fusion if you ask me.

Yeah, Nightmare was definately the toughest, followed by the spider.

Ridley wasn't nearly as bad as I imagined though, and the SA-X fight was simply fun

Ridley's pretty challenging if you don't have enough Energy Tanks to... tank... the damage.
He's nearly impossible to avoid because he takes up most of the screen, and it's only a 2-screen tall room.
The spider's not so bad if you can get the flame timing down and hide in a morph ball in the corner as much as possible.
I did a little better once I started to try to stay facing him so I can pound his face with missiles at point-blank range when he grabs Samus. At least if I have trouble wiggling free, I'm making sure his doing damaging her without opposition.

Nightmare really tires out my fingers because the only real way to avoid him is to space-jump in big circles most of the time. It's great if you can get an opportunity to grab onto the ladder and rapid-fire missiles at him. But the first time I beat him, I had to resort to constantly charging and firing charge shots at him while space-jumping around the room. Doing it that way takes something like 30-35 shots. Quite a finger-strain.

The plant boss was very frustrating until I realized if you fall in one of the plants, you have to simply hold A to jump back out (rather than pushing it rapidly, which is not successful). Just stand in place on the platform nearest it and pound it with missiles while concentrating on just ducking/jumping its shots as appropriate. The thing that makes it hard is that you'd probably have to farm for energy before the fight, because the SA-X probably took away a good deal of Samus' energy and then those big mosquitoes hit pretty hard.

Oh the plant enemy was very easy, actually I noticed that the flowers below do nothing to keep you there, they simply placed them in small water pools.
However, I usually have more problem killing the bosses x-cores (I usually get to that part with low health) than the bosses themselves.