So I bought Sonic CD on Steam yesterday (they just released it a few days ago), and today I played a few levels. Here's my opinion of it:
Gameplay: They haven't changed a thing. The gameplay is still as wonderful as when I played it as a child on the PC port. 10/10 for maintaining perfection

Visuals: The visuals have been taken to the next level. They are much crisper and clearer. But OH NO! The nostalgia factor has been ruined, you say? Well that's easily remedied. The game comes with a "nostalgia" option for those who like their blurry classic graphics with less blast processing. They even left in the anime opening! 11/10 for perfecting perfection

Audio: At the startup screen, I was slightly confused. Isn't there supposed to be an instrumental of "Sonic Boom" as Sonic wags his finger at you? Ok, maybe I remembered it wrong. Oh look, the best part of the game, the anime opening with everyones favorite song... an instrumental remix of Toot Toot Sonic Warrior??? What. The. Hell. ALL THE MUSIC WAS CHANGED! EVERYTHING!!!!! You don't even get the creepy Robotnik music. No Sonic Boom to be heard at all. Mind you, I'm still near the begining of the game, so MAYBE they have it during the credits, but I highly doubt it. They did, however, leave in the audio clips for past/future and the time travel sounds. -5/10 for raping my childhood