A great game - if you know what you're in for.
Besides, it was on sale for $10.
The graphics seem a bit washed out compared to the old NES, especially the first area of Brinstar, and even though I'm playing it on a Nintendo DS, the game still slows down when there's a lot going on on-screen. If it's graphics you're after though, I can't understand why you'd bother with this :D. The sound is still kinda catchy (Kraid's Hideout, Brinstar) when it isn't being horrendously monotonous (Ridley's Hideout, Tourian), or just plain strange (Norfair, in some crazy asymmetrical time signature) but again, what were you expecting for a 1986 NES game?
My only complaint (albeit a minor one) is the controls, which applies to all of these "Classic NES Series" games. It'd be nice to be able to map the 4 NES buttons to different buttons on the DS/GBA, especially select, which really gets a workout in this game, especially in the last area.
If you played the original Metroid back in the good old days and enjoyed it then, you'll like this game. It's not perfect 10 material, but it's much better than a 5.2. It's a fun game to have with you on the go, and for $10, you really can't go wrong.
I can't recommend it to anyone who first experienced Metroid on the SNES or later systems, though. If that's the case, better skip this one unless you like drawing maps (what we did back in the old days before the game did it for you).