Pokemon XD is a worthy follow up to the orignal Gamecube RPG.
The story takes place a few years after Coloseum's ended. You're a kid living in a labratory that studies shadow Pokemon and what they do. One day you learn about the ultimate shadow Pokemon, which turns out to be a Shadow Lugia. While the story is mostly cookie cutter, the latter part of the story might get you in. However, Pokemon is mostly known for its gameplay rather than its linear storylines, so it's passable at best.
Gameplay wise, its much more of the same Pokemon RPG game you've expect to come by in the series. The last addition to the gameplay, Shadow Pokemon, are back, but now they have a few new moves. Rather than have just one move (Shadow Rush), they have a variety of attacks, some aimed to both of your opponents Pokemon, or a move that halves all of the Pokemon on the fields HP. While it's useful, it's not really that original.
Graphics are slightly better that Coloseum, which is good considering that Coloseum was one pretty game. The designs are very detailed, and vary from town to town. Some of the character models look a little hokey, but other than that the graphics are some of the best that the Gamecube has to offer.
The sound is a slight issue. It's basically the same as Coloseum's, and the repetiveness is sometimes to the point of ad nauseum. To make it fair, however, the music in the game is really nice, and it's catchy as well, so despite it being awfully repetive, you'll be humming along to the tracks as well.
Pokemon XD is a lengthly game that will take you about 20 hours to beat, considering how good you are at Pokemon catching. I wouldn't reccomend this to someone who is new to the franchise (instead i'd suggest getting the GBA games), but it is definately reccomended to Pokemon fans who are looking to add a few new Pokemon to their Pokedex.