I also was looking at the Windham Weaponry Multi-cal. They had a article on it in the August NRA Freedom magazine.
This is one of those rifles that I really don't get. I'm of the camp that doesn't believe every rifle on earth needs to mimic the AR platform, even though I realize that the modularity of it is probably it's biggest draw. I think if I wanted a 7.62X39 gun and the AK was ruled out, I'd buy a Sig 556R, fondly referred to in my circle of shooting friends as the "swAy-K". That way the rifle is still piston driven, and more refined than my beloved Kalashnikovs.