I don't know #s, but there's MANY folks using it as a front end for HAI/Elk. Enough that one of the webinars in the sig is about using Elk with CQC. Not just for UI purposes either, i'm personally using it as I find the CQC rules engine to be an excellent complement & supplement to the Elk one. I've got perhaps 75 triggered events, 25 scheduled events allowing me to do things the Elk alone cannot.
I can't speak for the HAI bit as I use an Elk. That might be better answered on the CQC forums, as some CQC'ers that use HAI may not come around here that much (or if they do, read a 'cqc on sale' thread).