Fantastic, gameplay and chao, good for it's age - Could use a few edits.
If you're one of those who'll play a game for a bit, then abandon it, then it's not for you. If you're one of those who'll THINK that they've completed it and completely ignore it was made for high replay value, you may as well not buy it.
But if you're one of those who cherish every game you get and don't stop until you're certain you've unlocked everything, this game may be like a dream. With plenty of levels, an enhanced chao feature and plenty of unlockables, you will probably be captivated.
Choose your side, and feel free to switch any time - Got stuck on hero? Go for dark instead. If you get stuck on both, go to stage select, go and repeat an easy level, and then go give whatever you found to one of your chao. Fantastic.
If you complete the storyline on both sides, not only will you find a little secret, but you will also still have plenty of things to do. You can perfect your chao, and more!
Now, onto the chao - This was my favourite part of the game. Once you find the chao key in one of the stages for the first time, if you complete the stage successfully, then you will unlock chao garden with that character. You will start off with nothing but the chao lobby and chao garden, which upon entering the garden, then you will find two eggs.
These eggs will hatch into chao. You can give them chaos drives, animals and fruit, which will raise their levels and allow them to do new things. After three hours in the same garden as that chao, then they will transform, and if you treated them correctly, they could come out looking cool or beautiful.
You can then pit them out against friends or CPs in race or karate.
The chao area of the game is definitely the most detailed and releaxed area of the game, and the secret areas of chao is fantastic.
The game plays very similar to the original Sonic Adventure, except you automatically move between stages and cinematics, and will switch between characters. This is the story mode, where you can play as the three characters of each side - For hero, you get Sonic, Tails and Knuckles; for dark, you get Shadow, Eggman and Rouge.
Each character will have different abilities - Sonic and Shadow can use the homing attack and somersaults, and will have fast moving stages, usually with 'high speed' music. Tails and Eggman will be in their walkers and can use their machines to lock onto and destroy targets, usually with chaotic music coming between 'high speed' and relaxing music.Knuckles and Rouge can punch or kick, fly and climb up walls, and must find three items randomly hidden around the stage, usually with relaxing music.
Each character will unlock different gadgets - Some are required for the game, some are just useful, hidden items. Almost all are different, though most similar. However, every character will find a difficuilt gadget to get, though it is very rewarding - The single gadget each and every one of them is... ... ... ... I won't tell you, find it!
It's a freelance game, and if you get bored/stuck on one thing, just do another. It takes pride in it's almost permanent spot - My gamecube.
Don't think it's bad just because it's old.