I use my Jervens bag all year round sometimes more open than others it is in my top three items of kit the first being a knife the second a firsteel
It is the best all round shelter item for any conditions as long as you use it right. Remember it is not breathable and so must be vented. I have used it all it forms as a poncho, sleeping bag, shelter/basha etc.
It is versitile I have the mountain camo version but even in woods I have been missed except when I start to snore
I have used it in all conditions and weathers I have even demonstrate during SERE training within my unit and all the Bosses and people are very impressed when you get then to stand or sit in it for about 5 minutes and sometimes I have to force them out of it. The cost is the only downer but as long as you are going to use it, it is fully justified I know has paid for itself time over time and I will not part with it. I will upgrade to the exclusive model with both liners but I still will keep my thermo extreme as well
I know I will have use for it or it will get used by the other half if I ever manage to get out in the cold (if I succeed in that I may never see it again)