The Hotsac is a vapor-barrier liner, which is really only useful with nightime temps below 10F, some would say minus 10F. The main purpose of a VBL is to keep your perspiration from migrating into the sleeping bag insulation and, in sub freezing conditions, freezing inside the bag.
When Polarguard sponsored the Steeger Antarctic Expedition years ago, they said their polarguard bags slowly filled with ice but still kept them warm. They stacked their bags on top of the sleds and tightened the cover down on them. They couldn't easily be compressed any longer as they were that iced up. A down bag would have been useless.
Andrew Skurka used a VBL in his bag and even used vapor-barrier clothing when he circumnavigated Alaska (mostly in the winter) many years ago. That is probably about the best info available on the use of vapor barriers.
I don't see the Hotsac as being a good pack liner, though, on a winter trip, you could as it would be dual-use and would save the two ounces that a trash compactor bag (my favorite pack liner) weighs.