A somewhat difficult single player mode masks the brilliance this cart provides.
As a Sonic the Hedgehog game, S&K sticks to a tried but true platformer formula. Indeed, the gamer will note that little has changed from Sonic 3 in S&K. When looking at the history of Sonic 3's development those should come as little surprise to anyone. Essentially, the game is little more then a Sonic 3 expansion pack.
But the game has its own merits and can be played on its own without owning a copy of Sonic 3. As a Sonic game it is a little more difficult then previous titles. Without a dedicated save feature a.l.a Sonic 3 this game can give even the veteran Sonic player a bit of trouble. It is a little more versatile then Sonic 1 and 2 insofar as you are able to choose between taking on the role of Sonic the Hedgehog or Knuckles the Echidna. Each character has his own strengths and weaknesses so playing through both characters at least once will be rewarding in its own right. To the Sonic 3 veteran however, this may not seem particularly notable. Chief among the questions Sonic 3 fans might want to ask is: Where is Tails? He is nowhere to be seen in this game. With these things taken into consideration, S&K wouldn't necessarily deserve a high score. It is little more then a slightly more difficult Sonic the Hedgehog game that covers no new real ground.
Ahh, but that is where the genius comes. You see, S&K is more then a simple video game - the cart it comes in is more then a simple Genesis cart. Using a feature called "lock-on" technology, S&K serves as an adaptor that when combined with the previous Sonic the Hedgehog games provides a new and rewarding gaming experience.
In the case of Sonic 3, S&K can be combined to create the "complete" Sonic 3 experience. The content of S&K you see is actually material that was originally intended for Sonic 3 but had to be cut in order to make Sonic 3's release date. S&K remedies that and brings new life to Sonic 3. The game expands the possible save slots of Sonic 3 to a remarkable 8 slots and allows the player to play through the entirety of Sonic 3 and S&K as if it were a single game (which at its heart it was always meant to be.) This also allows for the use of either Sonic and Tails or simply Tails in the S&K content. This is essentially a symbiotic arrangement - both games benefit from S&K's connectivety and it is unlikely that once you own both carts you would play either one independent of one another.
Now, had this been all S&K had to offer then there may have been well-placed calls that Sega was simply trying to "double dip" by taking one game and splitting it into two seperate carts. Sega however decided to sweeten the deal by offering conectivity with other carts in the Sega library as well. When combined with Sonic 2 the player now has the ability to play through Sonic 2 as Knuckles the Echidna. This changes the game itself more then might be readily apparent given Knuckles differing abilities. So you'll be digging through your Sega carts to play Sonic 2 over again.
Now on the surface it seems we've covered all of S&Ks features - certainly the case only advertises conectivity with Sonic 2 and Sonic 3. But the programmers weren't done yet, they had one final trick up their sleeves to make S&K a brilliant cart. Now normally when S&K is plugged into an incompatible cart a warning sign comes up claiming that the cart will not work with S&K. However, when plugged into Sonic 1 the player can hold the A, B, and C buttons on this screen to unlock the Blue Sphere game. Blue Sphere is essentially the bonus levels from Sonic 3 - in that game you would complete these stages in order to collect the Chaos Emeralds. But it is a fun game in its own right, sort of a puzzle type game. And there are litterally hundreds of thousands of levels to play when S&K is combined with Sonic 1. The sheer number of hours that can be spent on S&K with Blue Sphere alone is worth the purchase.
Bottom line? Sega provided with this cart. They went out of their way to create a game that people would want to own in their library. Unless you absolutely detest the Sonic franchise this game is worth owning. Even if you only have Sonic 1 in your library S&K is worth the pickup as you're getting at least three games in one cart. And considering the going rate of Sega games these days you can get hours of fun for only a few dollars.
Don't let your Sega collection go without this title.