I really enjoyed mastering the three main gameplay styles and earning A-ranks. I'll admit that the Knuckles/Rouge emerald-hunting stages were not love at first sight for me, but they grew on me, and now I actually kinda enjoy them.
What I did not enjoy, however, were the chao garden mini-games. Victory in these games never come down to skill. Instead, they come down to luck and the amount of time consumed. I eventually grew bored and then used the infinite animal glitch to help speed things up. Even then, however, earning the chao garden emblems was an ordeal!
Worst of all, the 3D Green Hill Zone stage is pretty lame. The stage looks awful with its messy 2D pixelated background and its dated polygons. Even worse, the camera is all over the place in the stage, which eliminates the thrill of exploration, and even cause me to run off cliffs that I couldn't see! It was very tempting to unlock a fully 3D version of the most iconic Sonic the Hedgehog stage of all time, but the execution was done poorly. Overall, I don't think unlocking this shoddy version of the beloved stage was worth the trouble of collecting all 180 emblems. It's only worth it if the player enjoys all gameplay styles in Sonic Adventure 2, including the chao garden!
I agree 100%, those damn Chao gardens were the reason why I could never got all the emblems as a kid, I just didn't have the patience to earn those emblems(well that and my Dreamcast died), so I never got to unlock Green Hill Zone. I've been thinking of buying the game on Live and trying to get them all again.
I didn't mind the hunting stages too much once I got used to the locations, ironically I actually found it much easier to get an A rank in hard mode then in normal mode(in hard mode the emeralds are always in the same place, so it's easy once you devise the quickest path to get them all, whereas in normal getting an A rank comes down to luck more often since the locations are different every time).
Note- I played this game no longer than a couple years ago so it hasn't been that long since I played it last.