I picked this up from WiiWare a couple days ago, and it does not disappoint! The tech seems to be limited to about NeoGeo quality, though maybe a bit above that for some of the parallax effects. It's almost like they made their own arcade system with hardware customized for the game, then emulated it for WiiWare. It even has slowdown when enough bullets are flying around, just like the NES and SNES versions. I think that might have been a side effect when it was first made, and kept in Rebirth as a gameplay feature--slow-mo to be able to handle the toughest dodging events.
The music and sound is great. The voices seem to be running from a DPCM channel or something and have that classic scratchiness. The whole game, including emulation, seems to fit in under 22MB (if my math is correct).
Download it and support this type of project. For those of you who didn't grow up with this style of game, the best advice is that you're not allowed to complain about unfairness. (This is by design.) Throwing the controller is OK.
Konami, please use this engine to make a new Castlevania game.