I think this game is an improvement on everything the first game did besides the actual gameplay itself. The art style is the same as the first game but way more stylized and colorful which is exactly how I picture sonic as a franchise. One of my favorite things about the adventure games is that every character had their own genre of music. But in SA2 this leaked into the levels, so each level theme was made to fit the character then the stage. It really put you in the shoes of the character you were playing as.
The story is almost the best in the series in my opinion and shadow the hedgehog is an amazing antihero in every way shape and form. Everyone in the story is super in character, yet they all act realistically to serious moments (well as realistic as a blue hedgehog on the GameCube can get). That last scene on the ark is the most the series has ever made me feel something.
Now the gameplay is where problems arise. There's the speed levels, mech levels, and treasure hunting levels. Thew speed levels are definitely the best part of the game as they're fast, reward skillful play, and are replayable. The mech levels are slow and clunky, but they can sometimes be satisfying when you chain a bunch of targets together. The worst thing this game has to offer are the treasure hunting levels. There emerald pieces are randomly placed, the radar is horrible, and the stages are huge. These levels can last so long if you dont know what you're doing and it drags the experience down too much.
So yeah this game is very special to me and to many but it fundamentally has issues and if i only truly like 1/3 of the gameplay i can only vouche for the game so much before i start to deny things that are just true. I still love the game though.