Completely remade from scratch for mobile devices and no longer poorly emulated - The update is free on iOS and the game is released for the first time on Android.
Remastered features include:
Proper widescreen support (Not stretched)
60 Frame Per Second Gameplay (Smooth)
Tails and Knuckles as unlockable characters
Spindash move (Originally from Sonic 2)
Time Attack Mode
EDIT: Now available on Google Play (Android) - Store Link: