One of the best Sonic games so far. It's not as amazing without Sonic & Knuckles attached, but it's still great.
The biggest competition that Mario had in the golden age of gaming was definitely Sonic the Hedgehog. Although he never had a game quite as good as Super Mario Bros. or Super Mario World, Sonic is still a force to be reckoned with, and the third game for the Genesis proves this.
The gameplay is the same as the other two games, but it's just way more fun this time around. You can get various types of shields including bubbles, electric shields and fire shields to help you traverse each level. They all protect you from 1 hit before they go away, but the bubble shield lets you bounce quickly up and down and let you breathe underwater, while the electric shield attracks rings toward you and lets you double jump. It will also protect you from electric-based attacks. The fire shield will go out in water, but it lets you do a powerful and fast side attack maneuver. It will also protect you from fire-based attacks. Another new attack is that once you jump in the air you can hit attack again to hit enemies slightly furthera way from your ball (you turn into a ball when you jump) on all sides. You collect rings for "health" more or less, since if you are hit without any rings on you then you die. In order to get chaos emeralds (to get Super Sonic who is invincible and FAST. Once you have all of the emeralds and 50 rings you can turn into him. Your ring count will decrease until it hits zero (ending your turn as Super Sonic), but you can get more rings to increase your time as him) you need to find the huge golden rings in each level and complete the bonus stages accordingly. The big difference here from the other two games is that you don't need 50 rings and then a checkpoint to access these stages (a HUGE plus. In this game, having 50 rings and hitting a checkpoint lets you play a minigame to get extra lives and/or any of the three shields), AND the actual bonus stages are hard enough to be challenging, but not impossible to beat. In Sonic 2, for example, if you didn't have anybody to play as Tails then there is no way you would be able to get all of the chaos emeralds. In this game it is only Sonic who partakes in the bonus stages, and the stages themselves are like a huge grid on which Sonic runs forward forever until you change his direction. You can jump as well, and your object is to get all of the blue balls whilst avoiding the red ones. It's a fun minigame that doesn't repeat all too often (by that I mean that the stages themselves are physically pretty varied). The levels themselves are just right balance-wise (each with their own unique theme and music), although experienced gamers shouldn't have TOO much trouble with them. It's really neat that they all flow together this time ala Half-Life, instead of having separate levels like in the first two Sonic games. This adds a LOT to the immersion factor. The bosses themselves aren't too too hard, although the last boss can be pretty frustrating. Unless you are playing on the Sonic collection for the PS2 or what have you, you CAN be forced to start from the beginning of the game if you die too much (on the collection edition of the game, there is essentially no penalty for dying). The Genesis version is definitive in its difficulty (since you can't continue after dying ad nauseum unlike on the PS2 version), but otherwise both versions are the same. The only complaint I could say here is that you don't really need to completely explore each level like you would in Super Mario World, for example. There's little reason to unless you REALLY need to get to a bonus stage if you died trying to do the other earlier ones. All in all, it is a challenging game that, although short, will keep you entertained till the end.
The story itself isn't that different from previous games, it's just presented much better. Every level flows into one another, and you are repeatedly introduced to cutscenes with your nemesis Knuckles (a new character who is a perfect antagonist here) who constantly interrupts your progress (he stole your chaos emeralds at the beginning). Of course, he is just working for Dr. Eggman, but that shouldn't come as a surprise. Tails serves a minor role in this game (although he accompanies you the entire time), and I can't say I was disappointed.
The soundtrack is definitely one of the best in the series. The first two games had very good soundtracks - original and very memorable, but this game is slightly better (although nothing beats the mystic cave theme from Sonic 2 in my opinion). While it's no FF7 soundtrack, each stage's theme fits perfectly with the art direction and just sounds good in general. The hydrocity and carnival nights songs are my personal favorites. The sound effects are the same as in the first two games with a few new ones added in, and although the sound quality isn't as great (considering it's on Genesis), they are still memorable and work well with the action. Knuckles's theme is particularly cool.
The graphics are easily the best for the Genesis Sonic games. Each level has its own distinct theme and they are done very well. The graphics are crisp and clean and perfectly capture the cartoonish look of the franchise. In fact, I'd say the graphics are probably the only real improvement of note over the previous Sonic games, as the difficulty and gameplay aren't THAT much different. The graphics just take the game to a whole new level of coolness and immersion.
Except for the introduction of Knuckles, the characters are all from the previous Sonic games and still remain badass. They are still some of the best character designs in any game ever made (who doesn't know who Sonic is?). Knuckles is the perfect antagonist for Sonic with his red color scheme and cool-looking dreadlocks. Everything else is practically the same, from the villains to the actual monsters in the game, but none of them disappoint.
Although it has a few flaws, this is definitely one of the better games ever made. It's not quite as good as some of its Nintendo rivals, but it's a damn good attempt at taking the throne away from Mario or Zelda. Anyone who loves platformers or games in general would do a disservice to him or or herself not to try this game. Recommended!