I bought these boots because they promised to be warm enough to take on back country hikes in sub-zero temps.  I've had other "-40" boots that failed to keep me as warm as I had hoped.  This is my first experience with aerogel (vs. Thinsulate in other boots I've had), and will need to wait till winter for the verdict.  In the meantime though, I can tell that the boots are very comfortable and allow me enough room for good blood circulation (critical for staying warm), and at the same time still offer the support I need for use in hiking, esp. with crampons - which do fit the boots (BD sabretooth strap-on), with length set 3 notches longer than what I use with my mountaineering boots.  The support is much greater than I would have thought possible with such a lightweight soft boot.  After some exchanges, I found that I needed to go a full size up over street size to avoid toe jam in conjunction with extra heavy weight wool sox.  My winter gaiters can barely get around the 16" girth of the boot tops on my lower calves.  I'll definitely size up on next set of gaiters.  Though it wasn't one of my major selection criteria (so long as within reason), I am nonetheless impressed by the lightness of these boots in comparison with what I've been using over the past two years (NorthFace Lifty), while simultaneously they offer more ankle support and protection from the lumpy feel of MicroSpikes underfoot that I'd become resigned to with the Lifty's when hiking on hard, regular surfaces such as sheet ice on approach roads.