The Descent series has always a favorite of mine. I love FPS games, and I love the unique spin that this series puts on the genre. The games take elements of both the FPS and space flight sim genres while adding a touch of arcade simplicity.
Desecent 3 added a lot to the series. It has mission objectives, fully 3d graphics, voice clips, etc. I love this one because it is brighter and less gritty than the first two. It may seem almost garish, but the colorful graphics are very pleasing to the eye, and for the most part, stand up to today's games.
As far as i can see, people still play this game online, so if you want multiplayer you can have that too.
Overall, for the price and distribution here at GOG, this is a deal you can't miss out on. The Descent games were ground-breaking when they were first released, and they are still incredibly satisfying today.