Compared to SA, it was a disappointment. It's still a great game, but it could have been so much more.
The Sonic/Shadow levels were more complex than the Sonic levels in SA, and more fun because of it. However, the levels were also more linear and less open to exploration than in SA.
The Knuckles/Rouge levels for me were about as fun as the Knuckles levels in SA. They had their moments, and once you understood the layout of the level (which took a while near the end), they became rather fun. However, most people tended to dislike them.
The Tails/Eggman levels were fun for me. I liked the shooting systems, and some of the levels were really well-designed. However, the levels really fit Eggman better than they did Tails. It just felt really wrong not being able to fly as Tails.
What really killed the game for me was the lack of minigames. The only minigame was a superficial kart racing game. It was all too similar to the Twinkle Circuit in SA, only with a less interesting design. At least it was multiplayer, however. I have to give it that.
However, there were no snowboarding games (just a snowboarding section of a level), no pinball, no flying games, and no Hedgehog Hammer. There weren't any puzzles or adventure fields either. This did make the game more like a classic Sonic game (where you randomly move from one area to another at the end of a level), but it really did take the "Adventure" out of Sonic Adventure.
Overall, I'd give it an 8/10.
You know what would have been cool? If they had a feature where you could play through all of the levels of one specific game type in order of difficulty (i.e. City Escape, Radical Highway, Metal Harbour, White Jungle, Green Forest, etc.). It kept track of your score from one stage to the next, and added them up at the end of game, along with your total time. That would be just like a classic Sonic game, although a tad on the short side. I would constantly want to replay that in order to improve my time, or my score, or just for fun.