I've downloaded gigs of 3rd party maps and models for this game over the years. I'll play the heck out of it, then get sick of it for a few months, then go back and find some more maps and play it again. Best value of any game I've ever owned.
Came out around the same time as Quake II Arena, which I also still play. The "feel" is better in Q3A I think, but the multitude of play options in UT make sure it never gets old.