Sonic tries to do something that most of his games had problems doing, surviving the switch into the third dimension. Sonic Adventure succeeds in most aspects, but some of them are just completely unnecessary.
First is the introduction of hub worlds, in which the player can interact with people in Station Square and find out where to go. I really don't like hub worlds, especially in my Sonic games. Thankfully, there is a little light ball known as Tikal to guide you where you should be headed. If hub worlds were structured like this in future games, or were even smaller, I would be fine would that.
The levels are well designed and cater to each of the characters you play as. I just wish there were more Acts to them instead of playing them all just once. Almost every other Sonic game had two or three Acts to a level. You can play them again to complete other objectives, but it's just the same level, and nothing different.
You control 6 characters, each with their own part in the story. Sonic, Tails, Knuckles, E-102 Gamma, Amy Rose, and Big the Cat. The main trio control great. It's the other three's gameplay styles that really bug me. Sonic the Hedgehog is about speed. This is why Sonic 3 and Knuckles is my favorite game. Sonic, Tails, and Knuckles were all fast, they spin-dashed, and jumped into enemies, but each had their own type of gameplay style. Tails could fly, Knuckles could glide and climb walls, Sonic could use the power of the elemental shields. It all worked. But for this game, not so much. Amy Rose runs at a snail's pace, and she attacks with a hammer. She can get increased jump height when running at full speed to do a hammer vault, but her gameplay is extremely slow. E-102 Gamma's gameplay is nothing but shooting enemies with your gun attachment, which makes its boss fights the easiest thing in the world. Big the Cat, where do I begin with this big idiot? He's slower than molasses, and his gameplay is nothing but fishing for his Froggy. I'm not kidding. Who thought this was a good idea? What really is the kicker is that he has the most upgrades out of everybody.
The story kind of branches off and can sometimes be inconsistent, but the overall gameplay (for half of the characters), and great music make up for it.