I've spent a few hours with this, so this isn't necessarily my "final" impressions.
I didn't really go into this with any delusions that it'd top Colours, but I did at least expect Sonic's basic moveset to carry over. It's pretty distressing to find that underwater controls are reset back to being pre-Colours limited, and that wall jumps are no more, removing a big part of the freeform platforming that made Colours so lovely. Levels being more shallow again was to be expected now that the powers are gone with no quirks to replace them, but there are elements of levels where I expect to be able to move more dynamically, and simply can't with this moveset. That's a bummer, and I hope "Dimensions" rectifies this.
I'm surprised by how alright I find the classic Sonic bits though. After the demo's eye bustingly weird camera pans and wrestling with even his basic movement, the levels are generally built so you don't necessarily have to fixate your eyes on what's coming up. When you do, it's usually slowed down to a pace where it's more comfortable to look at, too. It doesn't feel exactly like the old Sonics, but as far as I'm concerned that's mostly a problem unless it's still fun. I think it's pretty fun. It certainly puts the now downright abysmal Sonic 4 into even less flattering perspective. I hope ep 2 learns like mad from this, or it might aswell be a flash animation of people blasting that thing with fire extinguishers.
Generations is jarring though, and neither game invites the other. Pressing that "Y" button (or triangle I guess) to switch Sonics always feels oddly difficult to me. When you find yourself in the classic Sonic groove, smoothly running through loops and bouncing on things at a methodical, pleasant pace through strangely intimate locales it's hard to feel the desire to go twitch reflex/split second decision/boost melting your face off with the modern stuff, and likewise it's hard to feel compelled to slow your heart rate after an exhilarating zooming through 3D landscapes with your avatar responding to your every flashy whim. Like many pre-colours Sonic games It's ultimately halves of two different games slapped together, and that notion is difficult to escape. In a way Sonic has become his own shitty friend.