I generally don't work on top of libs (aside from the std lib) as I tend to do very low level stuff, so "none"
I use Phobos, but that's because I'm lazy and that's all that was around when I started.
I'm using it in a commercial app. But we are very much in the early adopter segment.
Descent is really making good progress. However for the stuff I do (very heavy template work) it is not up to the task. However for most things that should not be nearly as much of an issue.