The game was HIGHLY inspired by Sonic the Hedghog 2 on Sega Genesis. Many of the enemies and a few of the level themes are pulled from Sonic the Hedgehog 2, they use the special stage from Sonic the Hedgehog 2, and the boss battle right before the final battle with Robotnik is against Metal Sonic. But the level layouts are different, you're ARE NOT playing the same game here. And you fight Knuckles, which never happened in Sonic the Hedgehog 2.
My one and only complaint, though, which was a little annoying, was the fact that when you get hit and your rings splash out of Sonic's body, it's trickier to get them back. For a second after you're hit, you simply can't pick up the rings, the game won't let you. And after that second is over, there's a very short period to get a ring before they disappear completely.

