This was my first Pokémon game. I played XD and its predecessor (Pokémon Colosseum) with my dad when I was 11 or 12, and I was actually a little underwhelmed when I finally tried Pokémon Ruby, my first mainline game, because this one takes a different approach that scratches the RPG itch better for me.
As opposed to the standard formula of having the player simply go to gyms, complete the Pokédex, and deal with some bad guy crew, XD (and Colosseum) puts a lot more emphasis on the story. Instead of completing a Pokédex "because potatoes", the player is given a (IMHO) more compelling reason to collect Pokémon: you're rescuing them from the villain team. In addition, the limited roster of Pokémon the game has encourages the player to put some Pokémon on their team that they may not normally use in the mainline titles. In addition, nearly the entire game, you get to do the normally-rare double battles- which also makes things a lot more interesting when it comes to strategy.
Knowing how overprotective Nintendo is of their IP (especially Pokémon), I will be VERY surprised if GOG actually gets this one- but it would be really awesome to be able to play this game again! It was a really unique experience that we don't really get as much of with the modern titles. They have some neat mechanics, too, but there's something about XD that makes it stand out more to me. It kinda bugs me that they never continued with this formula after this one, because I personally think this is a hidden gem.