Another fabulous entry into the classic franchise. Its too short and not innovative enough to replace Super Metroid as best of the series, but its still one of the best platformers you can get your hands on.
This time around, bounty hunter Samus Aran is trapped in a science facility where everyone (including Samus herself) has been infected with the mysterious X-parasite. You run through the station fighting the infected monsters. The game plays a lot like Super Metroid, but there are a few differences. Like Super Metroid, the control scheme is perfect, and the boss battles are hard until you learn the trick to each boss. Unfortunately, Fusion is smaller than Super Metroid and there are less weapons and gadgets, but movement has been improved a bit (Samus can now jump in ball form and hang onto ledges).
Also, the game is more linear (the facility's computer tells you where to go). Is this good or bad? Depends on your preference. If you liked the freedom and exploration of Super Metroid, its not as good. If you were annoyed by running back and forth figuring stuff out, than it's a huge weight off your shoulders.
Bottom line: Not one the greatest games ever made, but don't let that stop you from playing it as soon as you can.