Alright just got through running through this a few times and I have to say I'm slightly disappointed it was only a single stage and there wasn't a 3D stage (wanted to see how that played but oh well).
However what was here I absolutely loved.
I'll talk about the negative at first, or at least the things that stood out in my mind as being a little off. Sonic's jump (in my opinion) could use a little more height to it. I feel like I'm playing as Knuckles again and not quite able to jump as high as my blue counterpart. Sometimes his spin dash wouldn't work right, he'd bend down to do it and when I'd tap A he'd just kind of jump a little bit. However once I discovered X was a dedicated spin dash problem that issue went away (I think it has more to do with the layout of the stage, it seemed like Sonic couldn't quite get a footing). I really hope they can fix this by the time this ships because I've been playing Sonic from the get go and I'm not used to having a dedicated button like that. It almost feels like the game wants you to use that instead of the normal down and whatever button you're using for jump. Neither of these really take away from the gameplay experience either, once I adapted I was playing just fine and it only took a few seconds to get into the swing of things.
The stage was a little weird to look at when I first started off. The screen felt a little busy and I found myself being distracted by everything in the background and just how good everything looked (I was just recently playing Sonic 3 and Knuckles), but once I got centered in on Sonic I was good and screaming along at top speed like the good ol' days. A few framerate problems here and there, but this is a demo that contains a single stage so I'm not going to harp on it (plus this is the consoles we're talking about here).
The only other thing that got me was when you get hit and drop your rings. In past games they all scattered and continue to bounce in their original direction until you collect them or they vanish after a few seconds. Here they just scatter out slightly then bounce up and down in the same spot. Makes it quite abit easier to collect your rings after getting hit but also at the same time removes that panic attack of "OH CRAP! MY RINGS! MY LIFE!" as you try to decide which direction has more rings you can snag in the next three seconds. Again this doesn't ruin the experience or hamper it in any way, but I felt foolish the first time I got nailed by an enemy and took off left to collect rings that stayed pretty much where I got hit.
Now onto the good stuff. This plays and feels like an old school Sonic game. I was actually a little surprised to hear the usual chirp as a friend signed online and see the little update pop up at the bottom of the screen. Beyond the HD graphics and Sonic's jump height being a little off this felt like a Genesis Sonic game. From the music, the look, the feel, how Sonic moved, even his little annoyed look when you leave him standing still for too long. Plus their own little additions to things, like the new animations for enemies we already know, the added effects like when Sonic goes into his spin dash and puts up a cel-shaded looking dust cloud behind him before taking off. Sonic picked up speed in the right ways, handled perfectly well when doing the platforming sections. The level has several paths which is awesome in my opinion and a few times I found myself noticing something as I flew past and making a mental note (just like the old days) to not be moving so fast next time I came through there.
Really wish I could have gotten a boss fight in there but I can wait for that.
The music is awesome too, this was one of my complaints about Sonic 4 is that it just felt off. It didn't have that familiar Sonic sound to it and this time around the music feels spot on from what I've heard. Yes I understand some of the music is from the older games and reworked, but that is what makes it so memorable and gives it that Sonic feel. It's like how the songs in Final Fantasy and Zelda always stay the same, yet change slightly from version to version. This feels like a natural evolution to the series (both in music and gameplay), this is what Sonic 4 should have been and hopefully Sega goes back to the drawing board (read: Sonic Generations) to rework their movement system for Episode II.
Thus far I'm sold, though I want to see a little more (3D levels and a boss fight for each) before I make the decision to go in on this day one. So far it is looking likely and I highly enjoyed my time spent with this (I'll run through it again after posting this). Definitely worth it to check out for any old school Sonic fan or anyone wondering why people like myself still get a little excited at the prospect of a good Sonic game releasing. So check it out doods and doodettes, Sonic's back and you heard it first.