Really great demo, From what's seen so far I have a lot of hopes for the final product. Overall here are my thoughts.
Firstly the game plays great, at no point was I ever frustrated from things feeling slow or from anything feeling off. The game is fast and revels in the fact that it's fast. Even without boost I never felt like it was difficult to build up speed, between boosts and tricks it's incredibly easy to keep the game going fast and like any Sonic game should the better you get the faster you can go. I especially enjoyed many of the mechanics associated with tricks, one of my least favorite things from Advance 2 was how vulnerable you'd be directly after a trick, so it was to my enjoyment to see the bit of invincibility you get after landing. Another great touch from tricking I enjoyed was the tiny bit of verticality you gain from hitting the edge of a platform enough to raise you up after tricking into it, the more I got to trick the more I see the great intricacies into the system and I think it was well worth it. The only real criticism I could think of was that it took me a bit of time to get used to the controls and even on a 4th playthrough of the levels I still found myself constantly pressing the wrong button. Of course I don't think there's a lot that can be done here as every button has it's purpose, but the tutorial helps a lot in helping to get used to the game.
Next to talk about the level design it feels like a boost Sonic game in the best ways, every level really lends itself to replay to try and go fast and explore with each new try, I wonder if Advance and Modern versions of the stage will have more significant differences in the future, but I think giving a penalty of one letter grade to Advance mode is a really neat idea and adds some risk vs reward into the mode. The only criticism I can think of is that Radical Route could feel a bit busy as it was hard to tell where I was, and it was hard to tell what could and couldn't be stood on especially with the Eggman crates, and the rockets would take some finessing to get working sometime.
Boss wise, I thought the boss was ok. It's definitely the most fun I've had with a Sonic Advance 2 style boss, and I think that the game's mechanics do a lot to give you the tools to deal with that style of boss without the annoyance that came with the bosses in SA2, but I felt the way the homing attack interacted with the boss had a tendency to slam you into being hit and flying across the screen. I also thought that the secondary parts of Eggman's ship could've had a health bar that displayed as after repeated homing attacks I tended to break the part and then immediately slam forward as I would hit the button expecting to homing attack again. Also for Eggman's attacks when off screen a small indicator could go a long way in showing that missiles are coming and to get ready to dodge rather than having only a second to react before being blasted by something you couldn't see. But on the whole I thought that the boss was fun and I look forward to seeing what you'll do with other bosses.
Finally just to talk about some miscellaneous things, I really enjoyed the boss conversation as it made the boss feel more fun. I also enjoyed the set pieces in Radical Route the slow fall as the Robots charge out into the level and the rocket coming to pick you up, really made the level stand out.
All in all I think this game has tons of potential and I can't wait to see where the game is next SAGE around. Thanks for the fun time.