Though it's been a warmish fall season in Minnesota, I've had a couple of sub-freezing nights with the MW4 UQ on my WB Blackbird. (Yeah, I know it was too warm for MW4, but I just HAD to try it!!).
In terms of insulation, it's good, maybe even great. I had trouble with it wanting to slip out of place and not cover my feet or, at different times, shoulders. After some trial-and-error, I determined that the suspension was too loose and not providing the needed tension along the sides to keep it in place. So...a bit more tinkering and I figured out that following the printed directions EXACTLY didn't work on my Amsteel rope Whoopie slings. The lark's head initial attachment of the JRB suspension thingie was slipping when a load was placed on it with a resulting lack of tension on the JRB bungie cords attached to the UQ corners.
My solution was to make the initial attachment of the JRB bungie cord suspension to rope in the form of a Prusik knot of at least 4 loops (2 times through though 6 loops and 3 times around would probably be better) and tightening it a bit to anchor that attachment in place. That seemed to give everything enough stay-put oomph to keep the sides of the UQ under tension (and thereby "in place").
So, I don't know if others have experienced this...or may have a better solution. Either way, feel free to post what works for you (or doesn't work).