I played this earlier. Loved it! I'm one of the (few?) people who actually liked Sonic 4, despite the wonky physics and it not being a true classic sequel. But i will say that this feels like how you do a true HD remastering. Physics worked great, only a couple of minor issues with not staying in the spindash long enough and sometimes randomly popping out of it when you shouldn't. The camera needs a rubber banding effect as well, so that it pans further to the direction you're running (giving you a better view).
VERY solid so far though. With some polish, and original level layout, this should be an amazing fangame, and how you do an HD classic Sonic game!
For the record, the graphics are getting some criticism for being too colorful and cluttered. Or not being able to see where you're going. I don't get that. And i never had any issues with seeing where i was going. I hope they keep the gorgeous graphics.