Overall:
I like it! It's a little too heavy on the reverb, resulting in it getting muddy in places. For example, in Brinstar it loses the bass rhythm completely.
Norfair completely fucks up the time signature.
OK, I wasn't completely fair. It doesn't screw up the time signature. But it does defeat the purpose for the piece having the weird time signature in the first place.
The synth's decay is a lot slower than the NES, so what is an eerie pregnant pause in the original soundtrack is now just a weird extra beat. The melody lost its breathing room.
Ridley's Lair and Mother Brain are outstanding.