I found this game in a store around 9 years ago and I didn't have any idea about it. It was one of my best games. It's a good adventure on a very LARGE map with a lot of secret passages, traps, corridors and other. Lot of enemies and aliens to kill. It is very hard expecially because you don't have a map and you can lose yourself (not a little flaw). You can save your game and that makes it easier.
You can compare it to Castlevania: COTM for GBA, but I think Metroid 2 has a larger map and is much harder but has no replay value.