Honestly, I can't really profess to be that big a Sonic fan. I like and have played most of the 2D iterations of Sega's headline platformer series on the Genesis, the Game Gear, and most recently, iOS. But I'm not a superfan by any means, and if I've ever played Sonic CD before, I don't remember. I certainly never owned it (though I do remember renting a Sega CD with a friend and playing Night Trap and Sewer Shark over a few weekends).
All that said, Sonic CD, release just recently as a universal app for iOS, seems to me to be an exceptionally faithful port of what I do remember about Sonic, offering up colorful graphics, strong controls (performed with a virtual pad and jump button, and all of the standard complaints that come with virtual controls), and a great Sonic soundtrack. I do know that Sonic CD is heralded as one of the best Sonic 2D games, and this version of it comes off as a loving recreation of the original title. Especially on the iPad, these varied and well-designed worlds are big, bright, and beautiful to behold.
And Sonic CD for iOS also offers a nice mix of old-school spareness with modern features. There are achievements and online leaderboards, but they're buried in very old-school style menus and are out of the way as you play the old games. I recently said that the Grand Theft Auto port was the best console recreation I'd seen on the iPad, but Sonic CD definitely gives that title a run for its money. It's very well done.
Plus, the game's only $2.99 for the universal app, which makes this one a no-brainer. You do have to enjoy Sonic's frenzied platformer gameplay, but whether you're a Sonic CD veteran or never got a chance to play this version of the running hedgehog, this one's worth a look.