I love this game. I pre-ordered it and played it the first day it came out. Now, this is by no means a difficult game. It does involve some strategy and a lot of backtracking. That's Metroid for you, though. If you've played the classics, you know what it's all about.
This game is simply amazing, though. The sound effects and background music make you feel like you are right there. Very very trippy...
The game is fairly long. It can be beaten in a very short amount of time, but if you take your time and explore and enjoy it instead of rushing through it, you could put 30-40 hours into it.
The bosses are very interesting and some of them are fairly difficult. Nothing impossible, though. I consider myself an average skilled gamer, and I had no problem beating the game.
I don't know what else I can say about this game, because I know that my review is not giving it the praise it deserves.