This was one of my favorite games as a kid, even though I sucked at it.
Loved the fake "super graphics" for the time at the title screen.
It played like most 2D Sonic games do, and it's much better than Sonic 3.
Even though Knuckles had a cool gliding move, I like playing more as Sonic for some reason. But the excellent levels are worth playing twice.
One of the coolest things about the game was the attachable cartridge. If you plugged Sonic 2 or 3 in the slot, you could play as Knuckles in either game.