A huge improvement over the original
Well the thing to like about this is that it dosen't radically change the sonic formula. Sonics still fast, the levels are still fun, but the thing is everything SEGA did to this game to make it better. They could have just made some new stages and say"That's it" But they didn't. They introduced a lot of cool things such as spin dash, super sonic and tails, which didn't make it feel like Sonic the Hedgehog Directors cut.
The music in it, is top class, with bouncy tunes, slow tunes which just makes the experience a lot better. The special stages are fun and the best in all of Sonic and there are more zones (Although, most only have two acts opposed to the original's three). Plus the two player modes are a nice addition, as they are fun and allow two people to go through the adventure (and versus each other) together.
My only bad point? Tails is useless. He can't fly in single player so what's the point of putting him in in the first place? Also, the two player versus mode only has three zones and the special stage.
The Bottom Line
This is the definite sonic experience. If you've never played it, play it now. And I mean NOW.