Great fun so far! Having no real problems with the controls, and the resolution is only an issue with the HUD on the iPhone 4. I only experienced a framedrop once, and that was when I was thrown very high into the sky. Smooth as silk other than that!
The special stages are a but daunting at first, but I was able to get an emerald the second time lying down in my bed! So just takes the initial try to get used to what's going on. I guess the main issue is you're not controlling Sonic, but the level itself. Still fun though!
I don't like how the game doesn't seem to play through uninterrupted though. Each time I finish and act or special stage, I'm taking to a menu where I can (seemingly) select ANY zone and play immediately, even if I haven't 'unlocked' it by playing it through the main game. Maybe I'm doing something wrong.
Either way, I love it! Was worth the wait.