I often consider "Sonic Adventure" to be the first true 3D Sonic game. Where "Sonic 3D Blast" was an isometric game, "Sonic Adventure" was the first, for lack of a better way of saying it, "freeform" main series 3D game. It was also my first introduction to the "Sonic the Hedgehog" series. I still remember being 6 years old, getting the game for Christmas with my shiny new silver GameCube, and thinking to myself, "What in the world is 'Sonic?'" That said, I had many fond memories with the game as a child, and upon revisiting it, I find myself still enjoying it.
"Sonic Adventure" is still a boatload of fun to play, the visuals are still great after so many years (even if the models have shown their age), the soundtrack is one of the best I've ever heard in any video game, and there's a lot to do here. Beyond there being tons of content, there's some good variety here, and maybe I'm the only one who thinks this, but even the lesser characters in the game (Amy, Big, Gamma) are still pretty fun to play if you know what you're doing.
That said, the game isn't perfect. For starters, the cutscenes make for good riffing material (the models are "meh" and the voice acting is rough), the camera can be atrocious, and even though I mentioned that the lesser characters can be "fun," they are still pretty unnecessary. As a whole, the game could've benefitted if they just included Sonic, Tails and Knuckles and extended their stories with more missions.
All in all, "Sonic Adventure" is one of those games that I wanted to give an extremely high score to because of nostalgia as well as its standing among other 3D "Sonic" games, but unfortunately, it is flawed in a number of areas, which does hold it back from being a 9 or a 10. Even so, "Sonic Adventure" has, for the most part, held up very well, and it's still a blast to play even if you're not a fan of the 3D "Sonic" games.
Final rating: 8 out of 10 "Great"