I haven't used one, but I know somebody who had one for about a season.
I don't see them around very often. In fact, I think I've only seen the one being used. It was never very popular. I think part of the reason for that - and this is just my speculation - is that the headtube is pretty dang long. It wouldn't work for me, and more popular enduro/omnium bikes have short headtubes to do dual purpose mass start and time trial use. Plus, since the bike lacks any aero styling, one wonders if it just occupies too small of a niche.