I haven't had a winter to test my SS setup yet, but I do see the value of them. They are relatively inexpensive and would seem to be fairly effective to some pretty low temps with the addition of a SpaceBlanket.
My concern is in the longevity of the pad. It certainly does not seem 'pack-worthy'. It appears to be fragile in nature and likely to rip/tear. Again, these are only observations since I've not actually put it through a pack test yet. That and the bulk of a pad in my pack are my only reservations about the system. Warmth for price, it is a tough product to beat IMO.