I have nearly finished the second island.
Personally, whilst I do have minor gripes about things like the controls and camera etc, I do think this is the best Sonic has controlled since the Adventure games. I always struggled to adjust to the boost games, because Sonic controlled more like some kind of vehicle than a person (or animal, I guess) and I found the games just too fast. I liked them and obviously got used to it. Some of my issues with those games remain, such as Sonic's jump just not feeling right (it's too heavy and restrictive, as Hermit mentioned), but overall I think everything works much better than any other 3D Sonic game that has been released since Adventure 1 and 2.
I also agree with one review that said it is consistently pretty good, but lacks any real epic moments or highs. There's no Sonic being chased by a whale, or a monster truck, and all the cyberspace levels (whilst actually very good imo) are a bit meh after a while because they only rely on 4 different design styles.
The great big robots, mini bosses, titans and what not come close. They're mostly fantastic, but they're hardly Shadow of the Colossus quality. Good idea and some are cleverer than others in terms of their design.
I'm excited about where Sonic Team goes next with this. Could the next game take place on Mobeus and have more Sonic-esque world designs? Could they have small towns and communities to explore? Could they bring back the Mystic Ruins and other areas from past games perhaps? Could "cyberspace" levels be expanded upon and draw from wider influences? I don't know, it's going to be interesting, because it feels like thr next game is going to be the tough one. They built a solid foundation here, but now they need to take it to the next level without breaking it or making the core experience inferior.