Don't think twice about buying this game just because of the Gamespot score. Sonic's first 3D game, to me, is its best.
Sonic's levels are basically the 2D games converted to 3D, just as how you'd expect it. In my opinion, it is much better than any other 3D sonic games. Tails' levels are a race against Sonic. Knuckles' levels is basically an easter egg hunt for chaos emeralds. Amy's levels are kinda like Sonic's, but with a hammer. E-102's levels are kinda like a precursor to Eggman's and Tails' in Sonic Adventure 2, in which you shoot everything in a 3rd-person perspective. And Big's are the most retardedess (if that's a word), because all you're doing is fishing for a frog. Yes I said it, a frog. A FROG.
The levels are divided into 2 sections: Adventure Fields and Action Fields. Action Fields are your regular levels, and Adventure Fields are the places you go to, to go to the Action Fields... yeah pointless.
As exhilarating as the levels are, there are some problems. There are some glitches that are just annoying. There are some cheap deaths here and there. And the lip syncing is just hilarious, seriously find a cutscene on youtube.
There is also a great feature to this game- chaos. Chaos are the cute little guys that you raise, and depending on who you use to raise them, the chao's appearance will change. It's a fun little minigame to keep you busy.
Overall, if you're looking for a great 3D platformer for the Gamecube, and already played Super Mario Sunshine, you should give this one a try. Fans of the Sonic series just have a lot to dive into in this game. So far, this has got to be one of my favorite Sonic games, and even one of my favorite 3D platformers I've ever played.