Blu here, I have been playing Metroid Fusion for 3 days, and it was an amazing experience. At first the story line seems a little lame, with the cliché X parasites, but you will eventually get past the name, and understand how lethal they are. The game took me 4:30 hours to beat, but that is also because I only found 44% percent of the extra missiles, life, and bomb upgrades. They game in itself gives a good challange, and the bosses are old school Metroid hard, until you learn the patterns. OK, now online with the over view.
At first the X-parasites look like weak little bugs that your power beam could easily destory, but they soon start to learn, and become harder. Another twist is that the X are mimicking your destroyed power suit, and there is an X Samus walking around. You also learn that the enemies learn to fuse their powers, and become more powerful monsters. The bosses are usually tough battles until you learn their attack patterns, but then they are just average bosses. Each boss also contains an X core, which is one of two enemies. One is a spiked orb that you shoot missles at, then it flashes and chases you, then becomes vulnerable again. The second is a more defined spiked orb, that has a small eye that will open. You shoot a missle into the eye, and the orb will launch an attack back at you.
The weapons in the game get more powerful as you progress of course. You start with a normal power beam, but then will obtain weapons like the plasma, charge, and wave beam. There are also many suit upgrades you can get, but I will leave those a mystery. The best thing to do with this game is to buy it, and shove it strait in your GBA. I would advise not looking in the book until after you get most of the items, because it can act as a spoiler. So go out and buy Metroid Fusion, and become the ultimate bounty hunter!