The Good: Classic Sonic. It's just like it was on the Mega Drive, bar nothing. It plays beautifully, and looks fantastic to boot. Smashes Sonic 4 into the ground.
The Bad: Modern Sonic, and his stupid-ass friends. Silly bloody 3D sonic platforming garbage with annoying characters, annoying voices and their doubly annoying interactions with one another. Hard to enjoy. If you have the stomach to get past this, however, you WILL forgive the game when it throws more classic sonic your way.
Summary: A wonderful game until modern sonic rears his ugly head. Thankfully, intermittent use of classic sonic will keep you playing.