After having played the Dreamcast, GCN, and finally PC version of this game, I can say that this is pretty decent overall.
It has all the goodies that the Dreamcast didn't, such as the black market, hero/dark chao, and a reward for collecting all 130 emblems (not spoiling it). Of course, it also has the enhanced HD graphics if those are up to your taste (some prefer nostalgia).
E-102 Gamma (shooting stages)
Open World (Could be a con for some)
Tails accompanies Sonic in his action stages, and can be controlled by a second player to collect items.
Did I mention Tails isn't in a stupid mech?
Treasure hunting stages are more polished than SA2, and provide a better detection system.
The game feels fluid, and the controls work properly for the most part.
The goals of each stage are clearly explained
Excellent menu theme
Hidden emblems that can be found throughout the open world
Intense, action based gameplay (for most stages anyway)
Camera can be wonky (is a problem in all versions)
Screen resolution is somewhat awkward depending on your monitor
Chao cannot be transferred from SADX to SA2 (without the use of mods)
Button to pick up a chao is mapped to the same button that pulverizes it (spin dash)
No online capabilities, so no online bonuses like on the Dreamcast (again, modding may fix this)
Terrible voice acting
Open world is pretty mundane
No Shadow (I kid...)
Has this game aged well? Probably not, but that doesn't mean it's not fun. It can provide a sense of nostalgia for previous players, and still entertain newcomers. I still prefer the GCN version over this, but this is a solid substitute that can be worth your time.