So far my impressions with the PC version aren't good. I'm working with a slightly lower-end card, a 4850, but I'm still able to run most new games smoothly on High settings with some tweaking.
With vsync on, the game plays very jerky and I get some odd stuttering here and there. With vsync off, the jerkiness goes down slightly but the stuttering is still there and the cut scenes tear badly. The game doesn't play as smooth as the demo.
The framerates not so good either. On average, it says 35-40FPS, but it feels jerkier than the 360 version and during Chemical Plant zone, even when the counter was saying 35FPS, it felt more like 15-20FPS. Is there a way to cap the framerate at yet? I'd rather hit a consistent 30FPS then jump around.
I ran into a odd bug too. I played the demo probably over 10+ times and never had a single glitch. On my first playthrough with Modern Sonic, when I tried to homing attack a spring, Sonic stopped in midair right before hitting it and hovered there for 3 seconds before just dropping. Overall, I'm not too happy with it. If I had to choose again, I'd go with the 360 version. This is just my experience. Others may be completely different.