This is probably the only FPS game that I've ever liked. The fact that I could travel in three dimensions and turn myself upside down really messed with my head, but I liked it. I also liked Bioshock, but I liked it IN SPITE OF the fact it was a FPS. With Descent I had fun both just shooting and exploring. It's a great game.
Another cool thing is, that back then devs often delivered their tools (aka level editors) with the game. Which lead to many cool and crazy user created levels. As the level-editor was special in that way, you could place multiple rooms at the same place in 3D-space. So you leave a room, circle back through a cavern and land in a different room, although you were at the same place in 3D. That fucks with your mind.