Sonic & Knuckles can almost evoke tears from diehard Sega faithful. When the game was over, it seemed as if the few golden years the company had were over. Though Sonic 3 was a better game, you couldn't appreciate either unless you linked them together. Sonic & Knuckles was like half a game, but even then it was still great. The game completed the "Death Egg Saga," if you will.
The only true way to think of S&K and S3 is together. If I could grade Sonic 3 & Knuckles, I would get give it an 11 or 12. Sonic & Knuckles allowed you to get the elusive Super Emeralds in S3&K. It also let you play through the game as Knuckles. It provided countless hours of gameplay (especially if you are a hardcore fan). It isn't as good as Sonic 2 or 3, but Sonic & Knuckles definitely holds its own in such company.