This is a masterpiece and the best 3D Sonic game.
Story
Dr Eggman unleashes Chaos a creature that can transform and get stronger by eating Chaos Emeralds and it is up to Sonic,Tails and the rest to stop that from happening.
Gameplay: You can play as six characters but only start off with Sonic as you progress you get the chance to play as others. The cast is Sonc,Tails,Knuckles,Amy,Gamma and Big. There stages are all different. With Sonic you get to race through the level which is the best. Tails you have to fly and beat Sonic or Eggman,Knuckles you have to find the missing pieces of the Master Emerald,Amy you have to escape from a robot called Agent 0,Big you have to catch froggy his friend and Gammas is a shooting sort of levels which are fun. The bosses aren't really that hard and you get to roam around the Adventure Field. you can find upgrades for characters like the Light Dash for Sonic,Shovel Claw for Knuckles,Tails has a Rhythm badge lets him do rapid tail attacks and there are more. You get to do snow boarding in some levels with Sonic and Tails. The game isn't hard. You also see the first appearence of Chao which is nice you can have enter them in races. The reason I think that this game is good is the level design and it is very fun to play.You also get to do free missions on certain levels and they are sub games also like flying in the Tornado 2 to get a very high score and sand level by going through ring to and if you go through consectutive you get more score so it is a fun game to beat your score.
Graphics
The Dreamcast offered fantastic graphics the amount of detail in the game was amazing it looked better than anything you would have seen on an N64 and PS1.
Sound
The levels music is also very good and so is some of the character music as well. Sonics is nice as well as Tails. The bosses have music each character has a different type of song and music sound. Sonics is catchy and Tails is nice the others are not my favourites but the intro is fantastic.
Lastabilty: Should take you about at least an hour to do everyones mission but getting all the emblems is going to be difficult and that others replay value.
Overall Opinion
This is the best 3D Sonic in my opnion it just amazes me how they can create this masterpiece but most of the other Sonic games in 3D lately have dissapointed. A very good game and any Sonic fan would like to play this. The game is very good and if you like Sonic you should play it.