I guess I'm kind of late, but I wanted to reply to this;
I installed D1X a while ago and started playing Descent with. At first, it was great, but as I got further into the game, it started to get rather buggy.
The screens telling you about new robot types stopped displaying the text. The graphics for the robots on these screens would "smear" like the robot was turning in wet paint. Sometimes the levels would take up to half a minute to start. The little screenshots on the save menu were just garbage.
Of course all there are just incidental things, the biggest problem I had was that the game would start slowing down when sounds were played. After a reboot everything played fine and the game didn't have any slowdown at all, even when shooting continously. After maybe half an hour or so, firing the lasers caused everything to go into slo-mo so badly that the game became unplayable. Quit the game, close everything down, reboot, reload that same spot and the game would be good for another 30 minutes or so.
It's possible that it's the fault of my onboard Intel sound and the SoundMax drivers, but out of all the games I've run on this system, I've only ever found one other game that exhibits the same problem; Carnivores II, which is a Windows game, and even that one doesn't slow down to the degree that D1X does. Others I've played for hours at a time with no noticable problems include; Jedi Knight, Serious Sam, Half-Life/Opposing Force, Heavy Metal F.A.K.K. 2, Jedi Academy, Tomb Raider/UB (didn't know about Glidos then), Doom/Doom2/Final Doom (using the Doomsday engine), Voyager Elite Force, Halo, Thief Gold, Prince of Persia 1&2...