Sonic Adventure DX is a great game to purchase, and is definitely worth the money (I will talk about the Steam release in a moment).
The story is very predeveloped, and sort of blank, but with a new approach to how things work and happen.
Sonic's control is very great, although you can spam the spin dash almost as much as you want, the good thing is that the game punishes you for spamming it (like Sonic Generations does with boosting and spin dashing).
Tails is slower compared to Knuckles and Sonic, but once you look around the world, you can find the Jet Anklet, taking care of the problem. But the problem with it, is that you can go at really fast speeds with it when flying, and take for example Windy Valley, you can finish it without even touching the ground!
Knuckles control has been brought into the 3D world very well, but attacking enemies is quite hard. so I just prefer jumping on them or avoiding them all together. The emerald hunting is pretty simple since the radar was made very well. They also add some replayability via emblem rewards.
Amy is pretty slow and has very little reason to be in the game, and some of her courses can take forever, and there are so many parts where I feel very uncomfortable controlling her. She shouldn't have a story in this game, because it is only about being chased by a pedofile robot and protecting a bird with a chaos emerald.
E-102 Gamma has a pretty enjoyable gameplay, but like Amy, it has no meaning to the rest of the story. E-102 actually is pretty enjoyable for me because it sidetracks off of the main story which is bad, but is pretty neat! Meaning that I actually like playing E-102 Gamma! But it doesn't mean anything to the overall plot.
Last of all, Big... yes. Big was introduced in this game, and has a horrible plot, and no reason to be in the story. I know E-102 Gamma and Amy had a sidetrack, but at least they aren't as stupid as...
fishing. I mean seriously get this out of my FIRST 3D Sonic the Hedgehog game! Granted, if you can learn his controls, his courses can be pretty simple, but if you really want all 120 emblems. I will have to play each course twice; spending more time on them, and using some extreme dedication. It's sort of like the Sonic '06 of this game.
Overall, I really like the game! But only 2/3 of it which isn't really what I like, because if I want to play the game over, I will have to face the crap that the game throws at me which is I just want to avoid.
Now... about the Steam release, It's pretty terrible. You cannot change the settings via the config launcher, and the only way to change the settings is via the config notepad file. That's an extreme inconveinience (yes spelling :P) because I hate going through all of the different configurations or settings to find something that I need. Also, the game is played windowed at all times unless you configure the settings to set it at full screen, but this is no difference. The game always plays at a resolution lower than what you want. There is a purple looking backround that covers almost half of the screen. There are so many inconveniences in this release that it makes me want the GC version, or more to the point, the DC version which holds as the original title.