So, does it work? Sureinsulation is insulation. As you note, you can even add insulation by stuffing clothes between the layers. I wouldn't worry about your underside, as that insulation always get squashed anyway, which is why you need a good sleeping pad. As for your head, I'm sure the hood is plenty warm for zero degrees, but you can always throw on a hat.
As for moisture, how could a bag get wet when it's under 20 degrees? Any precipitation will fall as snow, so unless you're camped outside without a tent or bivy bag, the bag should stay plenty dry. I've used down bags in all sorts of snowy, sub-freezing conditions without much of a problem, although I will say that in conditions right around freezing, if it's snowing heavily, it's well-nigh impossible to keep everything dry. Still, reasonable camp-craft should ensure that your down bag stays dry enough to perform. Contrary to what I think has become conventional wisdom, down bags can actually absorb a fair amount of water before they really crap out on you.
My advice: Sure, give the Buddy Rad a try. You really won't go wrong.