Please, don't buy this game. Just rent.
Some things that made the first Sonic Riders cool were throw away. The anime style of the characters, the type skills, the attacks that the characters did in race... all is gone. Now we are come across with a game with poor gameplay, short replay value and broken AI.
For you have an idea of the stupidness that this game has become: Now, the type skills aren't responsibilities of the characters, but of the extreme gears. Tails, for exemple, can't fly if you don't equip him with especifc gears. Other stupidness of this game is: to use the type skills (to grind, fly or break obstacles) and other gear upgrades, you must collect and spend 20 to 70 rings. And for gears like Omnipotence (that allows you to use the three type skills), is not possible to stay with two or more different type skills simultaneously.
The mission mode is boring (few types of missions) and rewards you with a horrible extreme gear, characters with nothing especial (Amigo, Billy Hatcher and Nighs) and Super Sonic, the WORST PLAYABLE CHARACTER YOU'VE EVER SEEN ON YOUR LIFE!
The Gravity Control and Gravity Dive thing is pretty cool, but weren't used right. There are few shortcuts to use it and the tracks could be bigger and better designed.
The first times that i played SRZG, i literally loved. After have finished the story mode and played a little more, i concluded that SRZG could have been waaaaaay better. Because this, i recommend you to rent it first and spend all your will to play in a few hours, instead blaming yourself by buying it, like me.