A demo is now available:
http://sonicfanremix.com/
http://www.filedropper.com/sfrdemo
It works pretty well (I'm running it on an Athlon 64 3800+, 2GB DDR2-800 and a Radeon HD 5670 with 512MB GDDR5); the physics are quite similar to those of the original games. It supports six levels of graphics quality and, at least on my machine, a number of 4:3, 5:3, 8:5 and 16:9 screen resolution modes (although no 5:4, a pity since I'm on a 1280×1024 display here), starting all the way down from 640×480.