Play it just after 3 or with it locked-on for the full effect of the last true Sonic adventure...
It is still fast-paced, painfully colorful, and insanely addictive. And as an end-cap to the series, it beautifully concludes. But it isn't perfect on its own.
First of all, the story continues as the Death-Egg crashes into a forest and Sonic and the brain-washed Knuckles go hunting for Robotnik. This makes Knuckles a new playable character. And he's surprisingly useful. His special feature is being able to climb up walls, similar to Tails, but arguably more efficient. And Sonic is the same as he was in 3, quick, smooth, and really... uh... blue.
The levels are distinctly Sonic. Colorful, repeating designs and intense platforming in every one. The music is definitely up to the series norm, with high points including Flying Battery and Lava Reef.
But the game is short. At maybe 2/3 of Sonic 3's length, it doesn't feel like a full game. But the ending will take you so long to beat that it'll make up for it quickly. That being said, you need to save your game with Sonic 3 or you'll never be able to pull it off. When you lock on the two, this becomes the greatest Sonic game ever made.
Robotnik's contraptions are just as deadly as always. They lack some of the imagination that impressed us in 3, but they're adequate. The last few zones bring back some familiar foes (from Sonic 2, if you're curious) and the chaos emeralds are the main plot arc. Only in this game can you become Hyper Sonic.
Last Words: Any platformer fan, any Sonic fan, any game fan should play this game to see how the Sonic story ends, and enjoy the ride while it lasts.