I recently played the new Japanese Nintendo Switch demo of the game. I know that not everyone here has a Switch or hasn't been able to play a demo at a public event so here are some thoughts on control and the like directly from the horses mouth.
Modern Sonic felt a lot more stiff than in previous boost games like Colors and Generations but in a good way. In the times where I could actually control Sonic fully, Sonic felt awesome to navigate with. At a more slower speed, Sonic's turning radius is really small and I think platforming should be easy at slower speeds. At high speeds, Sonic is still able to turn, but at a slightly slower speed. However, the game sometimes wouldn't let you turn. Problem is, the level on the demo had a lot of forcing the player to stay in the center of the path. The game also gives no indication as to when Sonic can turn slightly or when he has to stay in a straight line (like the sign in Colors where Sonic could or couldn't drift). I really don't like this design decision and given the hundreds upon thousands of boost pads the level is littered with I don't think it would have been a problem to give sonic full control during the entire level. The boost felt a lot slower compared to previous games but I was never really bothered by it. Anyway, the parts where I could control Sonic were fun and I look forward to those parts of the game. Spreed running the level was really fun once I got the controls down.
The Custom Character plays a lot better than I thought it would. Sure its a little slippery to control but never did I feel like the mistakes were because of bad game design but rather because I couldn't get the control scheme to click in my head. I don't know why, it's more or less Modern Sonic which I had no problems with. Anyway, the Burst Wisp is really fun to use and I think it's the best of the three. Not only does Burst give a fun jump mode but you can also attack with it while running. The Electric-Beam-Thingy-Totally-Not-Boom-Sonic Wisp seems to be a lot less versatile compared to the Burst Wisp as it forces the avatar to come to a complete stop when finished attacking. I've yet to use the Cube Wisp but from what I've seen the Cube Wisp seems to be a momentum stopper too, making me not really want to try it. The grappling hook is... meh. I don't think it's used to it's fullest potential compared to games like SpeedRunners (which is really fun, especially as a Sonic fan). The grappling hook felt like a slower and slightly more glorified homing attack as opposed to a new movement option. In summary the Custom Character was enjoyable but not as much as Modern Sonic. I'm looking forward to playing the Avatar levels and hope that more Wisp powers control similarly to the Burst Wisp.
The only Classic Sonic level in the demo was the fight with Egg Dragoon. It was... good enough (which probably isn't that reassuring). The spindash and dropdash felt way too fast but that could be because of the confinements of the smaller boss room. Not much more on Classic Sonic.
The demo itself has a one minute timer for each level. Though I can understand the reasons for choosing to limit play time from a financial viewpoint, it's still irritating when the best parts of each level seem to be when you're kicked out of the play session. I think in the long run this has been another stab in the foot by Sonic Team executives as most of the general public probably won't like what they see in the demo.
Once Sonic and etc. get hit, the rings they have are gone forever. No more ring safety net. At first, I really thought that this controversial change to the formula would be really jarring and unnatural however as I kept playing the demo (specifically the boss fight with Classic Sonic) I realized that I loved this change. Though it gives rings a lot less weight (which might be given back if rings are used as currency, maybe?), I think the trade off for a harder difficulty is worth it.
The level design in the Modern Sonic stage was probably the best in the Demo (given how there's more or less only two levels, that ain't sayin' much). There was a lot more openness in the 2D section than I thought there would be. Most of the paths in the 2D section felt designed like a classic game, with the top path being quick to navigate through, the bottom path being slower and and more platform heavy, and the middle path mixing between the two. I can't however, say the same about the 3D sections which mostly consists of boosting in a straight line. The demo ends around the time when the 3D level seems to open up. Which is a shame and then some. The level design of the Custom Character was... boring? Novel? I'm not really sure how to describe it. The demo ends at around the train sequence, making the level seem really shallow as most of the beginning part is just a straight line and a wall.
I never had any issues with the Switch version. Frame rate was mostly smooth (at 30fps, which I'm fine with) and the only pop-in I noticed was from the shadows. Load times for entering the stage wasn't that long (quicker than Generations on console, though I could be remembering wrong). I'm quite impressed with the importing team. Sure the Switch won't be the most definitive version but it's not broken at a technical either.
Overall I had fun playing the demo and look forward to the full game.