I think reviews on a whole have been extremely unfair to this game. Yes, it's a port of the DC's main launch title. No, they didn't change that much.
First of all, let's start on the positive side.
Graphics, while looking quite dated, have been improved over the DC original. Character models have been completely rebuilt, textures are sharper and there is more anti-aliasing than before. The game also now runs at 60FPS, which does dip occasionally, but we'll talk about that later.
The audio side of the game hasn't changed much, if it all. The soundtrack is quite good. The main theme includes lyrics, which may seem unusual for a videogame, but it's pretty good.
SA:DX also features the classic Sonic sound effects like the famous "boop" jumping sound.
Sonic Team has added many new modes to the game. There is the standard story mode, a brand new mission mode where the player must accomplish specific tasks and very generously they have added 12 (Yes, twelve) GameGear Sonic titles as hidden bonuses. That's like another Sonic Mega Collection!
Gameplay is a mixed bag. It varies from character to character. Sonic and Tails have fast paced running stages where you must get to the end of the stage as quickly as possible. Tails' stages add a little variety to this formula by giving him the ability to fly and forcing him to race against another character (usually Sonic).
These stages, while simple, are a lot of fun. And make up the majority of the game.
Knuckles, last seen in Sonic 3 and Knuckles, returns to find the broken shards of the Master Emerald. His stages generally consist of searching a large area for the emerald pieces. It's a little tedious, but it's quite easy so the stages don't last long.
Amy's levels play much like a slow paced Sonic stage. She is on the run from a psychotic "badnik" (Robotnik's robots) and must get to the end of the stage without getting caught. These stages are quite long, but there's only three of them. And they're not too difficult as it's fairly easy to avoid the robot.
There is also a new playable character called "Gamma". He's a robot turned rogue and must rescure his trapped colleagues. These stages are consist of shooting, and are a lot of fun.
Lastly, there's Big the Cat. Big's stages require you to fish for a frog with a tail. No, it doesn't make any sense. And no, it's not fun. At all.
Completing all the stages for each character unlocks a final chapter to the game, but I won't spoil that. But it's a great final boss and brings the story to a close.
And here's where we come to the negative point of the review.
Firstly, while the game has been upped to 60FPS, one might wonder why they bothered. The framerate does dip quite often. I think they should have just kept the game at 30FPS to keep it constistant. Even if it is a fast paced game.
Secondly, the game is quite glitchy. Sometimes the player would be speeding down a highway and suddenly find that they've fallen through the floor to their doom.
Such things don't happen too often, but often enough for concern. It's a shame Sonic Team didn't keep the game in development for a month or two more to iron out the major bugs.
I don't mean to end this review on a negative note, since it's a great game and I've had a lot of fun with it. Getting the A ranks on each stage is satisfying, and unlocking the GameGear titles adds a lot of replay value.
I got this game at launch and it didn't disappoint me at all, however, since the general opinion of the game seems to be fairly "meh", I'd recommend that people give it a rent before they buy. Just in case.