They're great, best versions available.
Got good reviews too:
http://www.metacritic.com/game/ios/sonic-cd
Christian Whitehead's ports are amazing, but experience has proven that games not designed for mobile will never be as good as the original version. The Sonic games are bearable at best on mobile, no matter what reviews say. They're fine if you have a controller, but you shouldn't have to use unusual input methods to properly experience a game.
And that's exactly where Nintendo is coming from. We may see Mario, we may see Advance Wars, we may see Rhythm Tengoku -- but we won't see the existing games. We'll see versions designed specifically for smartphones and tablets.