It's just classic 2D Sonic on GBA. There's a couple new moves in Sonic's arsenal, but nothing as game changing as homing attack or the dash boosts. Tails and Knuckles also appear with their Sonic 3 kits. And most interestingly, Amy also appears. Not only is Amy included, but she has by far the most unique playstyle, not having any spin dash or indeed any type of "ball" moves, relying solely on her hammer. This basically makes her the hard mode of the game, and while I don't really think her play style fits well with Sonic (not being able to turn into a ball during a fast section massively impacts the game since you have no safe way to get through enemies), I do commend them for putting effort into making her truly special instead of just a clone.

My one complaint about this game is the damn special stages. In concept they're not bad, but there's no depth perception, and no way to really tell the exact location of the rings and your character, so lining yourself up is next to impossible. Possibly the worst special stage I've played in a Sonic game.

Super short too. But it's just fun Sonic gameplay.

Post review edit: After playing through the game with each character and getting all the Chaos Emeralds, the special stages did seem to get easier. I found focusing on just the board helped to line up right.