Freaky and terrifying, but that's what makes it good
As far as games with Gamecube graphics go, this is way high up there on the sci-fi horror game ladder. And I say horror, because the game is structured in such a way where it simulates YOU walking through an alien wilderness, all by yourself, with no friends and no help, with only your super-suit between you and a horrific claustrophobic death at the hands of a mutated alien monster.
I honestly wish there were more games like this. There are no other protagonists beside yourself and no other characters to talk to, just you and a wide world (of horrors) to explore. The plot isn't handed to you cutscene-by-cutscene the way an RPG would share a third-person story with you; you have to scan your environment, use your scanner to hack and steal records from some of the antagonists, and all in all figure out what's going on by yourself.
The most terrifying levels are the ones, especially in the space pirate base, where the lights are cut out. Don't... play that by yourself in the dark. Just don't. The water levels, as claustrophobic and terrifying as they are, are easily the most beautiful in the game (despite the crap Gamecube graphics -- HD remake anyone?). When you're in the crashed Space Pirate vessel and you're floating down platforms to get to the bottom of the central section of the hull, watch the little schools of fish flutter about your helmet. Even the scythe creatures with one eye in that level look a bit like swaying seaweed. It's dark, it's ambient, it's gorgeous.
The music, I think, is one of the main factors in the game's ambience. Especially the water theme. Hypnotic and lucid, the theme puts you into another world.
All in all if you're into sci-fi horror, then go hunt down this game and play it. I demand you to do so.
We should all campaign for an HD remake of Metroid Prime. :D