If you are a fan, you'll like this game. If you are not a fan you won't, simple.
The graphics in this game seemed to have been reused from Sonic Heroes, with some minor enhancements to them making them smoother then they were before. Some enemies though seem to be very blocky, like the Black Arms, who in one cut scene seem to look like they were taken from the dreamcast. Although this might of been exceptable in earlier gamecube games back in 2003, it is not good now.
The controls are very loose. You can't stop well, and you take very sharp turns, which is not good. Also you immediatly go to full speed. That is bad as well. Also the gun has no lock on. Shadow auto aims to people sometimes, but it may be the wrong target, like if you are trying to do a hero mission you shoot a GUN solider by accident. The guns are fairly well developed though, and it makes the game easier due to the large amount of enemies that are in this game.
Sega has replaced the voices of characters as well. If anyone has watched Sonic X, they will immedietly notice "Hey, those are Sonic X character voices". This can be good, and this can be bad. For characters such as Tails, it was a much needed improvement. For characters such as Charmy, Vector, and Knuckles, it was a drastic dissaprovent, especially Knuckles, who's voice sounds so bad that you want to shoot the television when you hear his voice. As for the music goes, it is pretty good, rock type music, sometime you have come to expect since Sonic Adventure 1 came out back in 1999. Some music is good, some is okay, some is bad, but there isn't too many bad songs, so it is good.
The idea of having the choice to take different routes increases replay value to a point. If you are a hardcore Sonic Fan, you will keep playing until you get all the endings and all rank A's and all that unneccesary crap like I will, but if you are not a Sonic Fan and you pick up this game, you might play 3 levels and then give the game disk to your dog to eat. The story is good. You have 10 different endings, a last story, and 326 ways to get those 10 endings all together. Alot huh? Although no one will play that long, unless they do nothing for a year and have an ungoddly amount of free time on there hands. Overall, the levels themselves are fun. Although many blind objects such as lasers in level 1 can really make you mad. Also, it is the return of a bad game camera. This time you use the C-stick, but it still sucks.
Also, the gamecube version is the only version to have co-op mode. The second play can control your partner, such as Sonic, Tails, Knuckles, Rouge, etc. (you can't control Doom's Eye, Eggman, or Charmy to my knowlage though). You can't do this in the Xbox and PS2 version though.
Multiplayer is not that fun. You pick a Shadow, and kill the other. Not as fun as it sounds. Because of speed and no lockon, you will lose the second player easily.
The game is a simple side project that Sega made. They cut corners, used bad models for the characters that were not done in heroes, and they rushed through reusing game art, level desgins (all the ark levels, one of the ARK levels takes you through the areas that you went through in SA2 in final rush as Shadow. You will recognize some of the paths and such to the point that it is kinda wierd), and even music, such as they reuse the Eggman Theme (well, a remix), that city level you play as Shadow in SA2, and some other event music as well. It is a side project and nothing more. I personally love the game. I hate multiplayer and I see the games faults, but I still really like this game.
Overall, if you are a fan, you'll love, if not, you'll hate it. Simple. So I think that this game deserves a 7.5. Although I like it, it has it's faults, and this is what it deserves