NO SPOILERS
I had a LOT of fun with this game. As somebody who has always enjoyed the Classic Sonic formula, this was a fun experience from start to finish. If you enjoyed Sonic Mania, you will enjoy Sonic Superstars.
First of all, let me assure you that Sonic Mania's physics were translated over to Superstars very well (I'd say it was 99% carried over). Sometimes, however, I would hold onto my momentum even after colliding with a wall, but that's about it.
This game definitely is not perfect in terms of performance. I did encounter quite a few bugs on my first playthrough (hopefully they will be patched). Sometimes the music wouldn't play right, sometimes the music wouldn't play at all, I got soft-locked in one of the special stages, and the game did crash twice on me (that might be because of my Switch though, not the game).
The multiplayer was chaotic, and I mean that in a good way. It was funny watching the camera try to figure out where to position itself, and I was laughing at the madness on my screen! However, I could tell that the levels' designs and gimmicks were not assembled with co-op in mind, and that makes me wonder why the multiplayer feature was a selling point. Still, it is fun to race through stages with friends.
The music was ... okay. Don't get me wrong, Tee Lopes and Rintaro Soma did a spectacular job with the music (I actually set my controller down on a few occasions because I couldn't help but dance). Jun Senoue's music, on the other hand, wasn't anything special. The melodies of each of his songs were really good, but it would often be made bland or unengaging by questionable synthesizers or instrumentals.
Like I said, if you are at all a fan of Mania, I recommend Superstars. Overlooking all of the flaws, it is a fun time and a good Classic Sonic title. If any of the issues I pointed out are at all alarming, I would wait for a price drop or at least an official patch from Sega.
I enjoyed this game and I can't wait to see where the series goes from here!