The last good Sonic game until Sonic Generations.
The game is fantastic but not as good as Sonic 3. Sandopolis zone just plain sucks. I HATE that level. It's hard and not fun at all. If you get all the chaos emeralds you can play the Doomsday Zone level which just kicks ass. The music in the level is amazing and plus think how cool this is. You're Super Sonic flying in space, dodging missiles and destroying a giant robot.
Sonic & Knuckles is great if you have it on the Genesis since you can put Sonic 2 or Sonic 3 on top of the game to play Knuckles in Sonic 2 or Sonic 3 & Knuckles. On it's own it's a good game but just not as good as the other 2 games. It still beats Sonic 1 though. I still say pick it up. It's worth playing.