The X series is great. I've been playing it since the original first version. X3 Reunion is definitely the best of the series and expands on the game quite a lot, being able to own whole solar systems and amass a large network of ships that you can automate to get on with things like mining, while you enjoy getting on with other stuff.
The only thing with X3 is that it moves away from the original game's ideas. I loved the start of the first game, with you testing a new experimental spaceship for Earth and an accident sends you half way across the galaxy... Farscape like... into a universe that is completely different, and the first thing that happens is a large ship looms down on you and you are contacted by a strange alien race who help you out with some initial upgrades to your ship.