I have over 200 hours on my SA2:B file, no lie. I managed to miss the Dreamcast, so the GCN incarnations of these games were my first in-depth experiences with the games. And they were my grand reawakening to the world of Sonic. You see, Sonic 2 was the first video game I ever played (I was a huge bookworm during my younger years, and thus missed the 8-bit generation). Bought up Sonic 1, 3, and Knuckles, rented 3D Blast, became horribly disillusioned with Sega after I saw the Saturn die in record time, and then went Nintendo and never looked back.
But then, of course, a decade or so later, I did look back. Anyways, my opinions...Sonic Adventure was horrible. What a crappy game. Erratic camera, jumpy controls, and glitches out the wazoo. Even later on in the game, Sonic's levels were nothing more than "hold forward for 30 seconds, jump, repeat". Some of the other characters were fairly fun, but the utterly pointless overworld hub was poorly implemented and largely empty. That being said, it did have a few redeeming features, such as it's cheesy but very enjoyable story and music. God, I love Sonic music, 2D and 3D alike.
Sonic Adventure 2, however, was almost perfect, to me. Great (and still delightfully cheesy) storyline, a less broken-to-all-hell treasure hunting system, and grinding. A simple addition, but it added so much creativity to the level design. I loved the hell out of Final Rush, and found pretty much everything there is to find in that level. Plus, it introduced Shadow and Rouge, two awesome characters. Admittedly, Shadow's awesomeness was tempered somewhat when they brought him back and devoted the next two or three games filling out his backstory. But Rouge is still great. Secret agent treasure hunting bat of kickassedness.
I'll probably rant a lot more later, but this post is long enough as is.