Underrated; have to get into it.
Sound/Music: Most of the sound is pretty quiet, no nails-on-chalkboard annoying effects at all. Most of it is just when you hit something, jump, shoot or get hit, or of course die. The music is very good, especially the Norfair and Title bgms. Of course, the song you know most from the SSB series, the Brinstar theme, is in this as well.
Difficulty: Hard. Pure hard. There is nothing easy about this game, you're gonna die quite a bit. Everytime you die, you start with 30 health, and there's some enemies later that take 20+ HP from you. Not very easy there. You're also given no map, so basically you're going in circles and getting lost, so I would personally recommend to either 1: Get a map from Google, or 2: Draw one of your own.
Gameplay: So your objective is to infiltrate planet Zebes and destroy the mother brain. Sounds good. It's not gonna be easy to get there. There's tons of twists and turns you need to wind through. There's also a couple of bosses thereabouts deep in the underground of Zebes. Along the way, you'll collect missile upgrades, collect the morph ball and the bombs, and then new beams. All of these will be vital for you at many points in the game. In the bitter end, you will fight and destroy the Motherbrain, then you must make your escape..
Controls: Very simple, much more simple than the game. Start is pause. A is jump. B is fire. Left-right moves you left-right. Up looks up. Down uses morthball (use B to drop a bomb while in this mode), and Select toggles between missiles and your beam.
Overall: Many reviews have said this game is "crap". I don't think a lot of these people really took the time to get into the game and actually get through it. Of course it's not gonna be like the modern Metroid games, like Metroid Prime. But the game on its own has a lot to say for itself. It is very hard I admit, but the difficulty is worth the good feeling you'll have once you beat it.. If you can.