This game is good, and it's faults are more acceptable for the time than the 2003-2009 games.
The first thing that anyone will tell you about Sonic Adventure is that
1. It was amazing at the time.
2. It has aged.
I am not going to dispute either of these points. Sonic adventure has six different gameplay styles. However, In this game there is a clear difference in how they are presented. In later games it would be attempted to make these into equal sections of the game. However, in Sonic adventure? Sonic's gameplay, the MAIN gameplay as you will, dwarfs the other character's in playtime. They are clearly supposed to be side characters.
Sonic's gameplay is essentially a 3D distillation of the Classic series, but with a few new additions. Sonic can run, jump and spindash. There are also a few new moves, particularly the homing attack, a move designed to help 3D gameplay. Sonic runs through wacky 3D worlds rife with dangers, platforming, loops and some particularly memorable set-pieces. These levels succesfully emulate the feel of the classic series in a 3D enviroment. There is speed, but also wide open areas to explore, and hidden item boxes. The levels feel fantastical and surreal, and do not usually try to emulate reality. And even when we get urban city levels, there is a bizzare twist to it. The game's graphics are fantastic for the time ans the music is funky and overall good.
Tails's levels are fun, racing levels in the same style as Sonic's. However, the flying ability opens up many new paths and is very fun to use.
Knuckles's levels are simple, fun and exploration based games,I never found them particularly difficult.
Amy's are inoffensive platforming levels, not great but not bad either.
Gamma has shooting levels. Fun, and a nice break from the main platforming.
Big's levels involve fishing.Crap.Thankfully this does not take long and is only a small section.
Much has been said about this game's camera. Personally, I don't think it's very bad, most of the time. This might be a bit controversial. However it DOES occasionally screw up, which is annoying, but little more than that. Even so, in a 3D debut game like this a bad camera is infinitely more acceptable than in a game made in the middle of the 2000's.
Overall, I think this is a good game, especially for the time. Of the other playstyles 3 are fun and the 2 that disapoint only make up a small bit of game.