This is hands down the best Sonic game ever made. Even Sonic Rush and the Sonic Advance games can't beat it. This is a Sonic game everyone must purchase, and with lock-on technology, you can have the complete Sonic 3!
This Sonic game also has the best Sonic zones ever. You've got the Lava Reef zone, which is a cave with lava filled obstacles, and the Death Egg zone, which is Robotnik's flying base. Finally, the Sandopolis zone, which is a zone in the desert, and on the second act you're in a pyramid full of traps and dangerous ghosts. How cool is that?.
Not only that, but you get to play as Knuckles, which is probably one of the best Sonic characters ever made. He can climb, glide, and be played in almost all of the Sonic stages, even the Sonic 3 ones! Yes, that's right. Tails can also be played in the Sonic & Knuckles stages as well. I also forgot to mention that Knuckles can also be played through the whole of Sonic 2! So if you have your old Sonic 2 cartridge still, then just place it in the Sonic & Knuckles cartridge and you'll be able to play through every stage with Knuckles on Sonic 2!.
You can't really go wrong with Sonic & Knuckles, even when you don't have Sonic 3. You've still got some of the fast Sonic moments, and the coolest characters in it. If you really want the ultimate experience, then make sure you have Sonic 3 & Sonic and Knuckles. You won't be disappointed.
If you are desperate to play this game, then you'll need to buy it, but it's not easy to get hold off that way, so I'd suggest that you get Sonic Mega Collection Plus for it. Not only do you get the best Sonic game on it, you also get the other best Sonic games on it as well!.