This is pretty good for people who havent played Sonic Adventure DX for GC. Sega is making this game for the PC (yes, including the game gear games & everything else that was in the DX version).
http://www.sega.co.jp/pc/sonicdx/trial.html
You can only play as Sonic, Knuckles, or Tails. Sonic can play in Emerald Coast, Tails in Windy Valley, and Knuckles in Red Mountain.
Oh, the download is about 69MB, but it is a pretty good download, free & legal.
Also, graphics tend to be better on the PC than on the systems itself.
Oh, BTW, almost everything is in Japaneese in the game, and installer, so beware.