This port of Sonic Adventure... it's very buggy, and the graphics aren't quite as good. And that brings the game down.
Where did it all go when this game was ported to GameCube as Sonic Adventure DX? Well, the story is still there and all the levels intact, but let me assure you, this game is not nearly as good as its first incarnation. It's bugged to high heaven. You can run up try running in one direction when the camera pans and you end up going in another. You can try running along a slope like one in Emerald Coast and find yourself falling off because you aren't running at the exact point you are supposed to.
I have a soft spot for this game, mainly for having not played the Dreamcast version. But I have seen the Dreamcast version and know the graphics were knocked down a tad. IF you played Emerald Coast in both stages you know the water looks incredible in the Dreamcast version, but it looks dull and thrown in on this version. The game looks fairly empty, too. The hubworlds have maybe a few people, all mostly useless, and everything else is just boring expanse. It's like there wasn't even an attempt made to make the game look alive.
The story is what pulls me in. As poorly done as this game's dialogue is, the actual storyline is a great way to say "Sonic can do stories". The story involves Dr. Robotnik, now given the nickname Dr. Eggman by Sonic, wanting to use the Chaos Emeralds to power up a creature that has been awakened called Chaos. Each emerald given to Chaos alters his form, and makes him more powerful. This leads to numerous bouts with Chaos with the 6 playable characters.
Moving to my next point, there are 6 characters to choose from. At start you only use Sonic, but playing through his story will unlock the other 5. You can play as Tails, Knuckles, Amy, Big, and E-102 Gamma. Most of the gameplay is pretty entertaining, but looking at the stages Big and Knuckles have to get through, and to an extent Amy's as well, not every stage is a lovely walk in the park. With Big's stages and Knuckles' stages, these become a walk down a foggy street in the middle of a thunderstorm and your feet are frozen from the cold and you just wish you were in a nice room somewhere to warm up. Big's fishing stages are hands down the worst addition to the Sonic series, even with bad gameplay from future titles like Shadow and Sonic 2006.
There's also a new Mission Mode, and you can unlock Game Gear games for play, but these two additions aren't that special and are really not worth going after. In fact, the Game Gear titles you can unlock are playable through Mega Collection and Gems Collection, so you're better off getting those.
The sound, I have mixed reactions to. The background music, while not the best, at least helps aid the mood of the level you are in. Some levels have multiple "themes" as they are split into parts. However, the music has a sort of reminiscent quality to it, almost making you think of the older games' levels like Sonic 2 ans Sonic 3. The voice acting, I cannot say is very good. In contrast, it's actually poorly done. Dr. Robotnik's voice is fair, and Sonic's is in character at least, but most characters' voices either are absolutely annoying (Big) or are tuned out by the BGM that plays at full blast even in cutscenes.
The game has a nice sense of speed, but with the bugs you encounter, mostly when you are moving and the game pushes you to one part, mostly when you are running and the camera is moving. This speed really only comes with Sonic and Tails, though, and all the other characters feel somewhat slow. Amy is the worst here because she is just not made to play through a real, get-to-the-end style level. Big is clumsily slow, but at least you don't actually move him much. Here's where the paradox comes in: how do you have a game that's supposed to have speed, when one character has the absolutely contradiction to such a concept (in this case fishing is our contradiction)? Barring the Big and Amy stages, the levels had a decent flow to them.
All in all, this game is basically the beaten identical twin. It was frowned upon by society while its twin rose to great success, but because this twin was so unruly, it was shunned and no one ever liked it as much as they wanted to. This analogy is as close as I can get to comparing this game to the original Sonic Adventure on Dreamcast.