While unable to test in the extreme conditions I had hoped (summiting Mt. Washington mid-January and due to absolutely Everest-like conditions), I have spent some quality time with these Rs throughout the coldest December/January Ohio has undoubtedly seen in the last few years
Being a thin, six-foot bean-pole these thermals in size medium did not fit overly tight for a base layer. The elastic waistband is just barely tight enough for a 30 waist. They are intended to be snug, but not tight not as tight as thermal underwear. The mostly spandex material is light, compressible, and quite comfortable, while the raised and checkered texture gives you something to think about the first few wears. It is a bit odd, but provides for superior loft of the fabric above your skin to provide very good insulation. The mens version comes with a rear zipper pocket that might be useful if using for cold-weather running, but could probably be done without. More importantly is what the pants should have - a zippered fly with an insulating flap. You dont want that area to freeze for sure, but it would be nice to be able to keep these very warm thermals all the way on when going about business, youre already stripping off outer layer(s) as is
And boy are they warm. While climbing in Arkansas this past December these never came off. Standing around the crag in low/mid 40s and wind my legs were never once cold with these under a light hiking pant. The pants are also extremely flexible and stretch really well, no pulling whatsoever. Back in Ohio hiking with 30 or so pounds to train in single digits (but no wind) I actually had to zip off the bottom portion of my pants I was so warm. Running in low 20s and wind with just running shorts over (I still cant wear totally form-fitting I suppose Im a self conscious bean-pole) resulted in a similarly sweaty outcome. They are, however, surprisingly breathable and dry very quickly due to the R1 fabric technology.
All in all these are great thermals, but just maybe not great pants. From my experience would be best for very cold weather in active use or medium cold weather if planning to be static. Patagonia made this superior piece of clothing to keep you warm, dry, comfortable, and alive - and the R1 technology does just that. No frills, no extras.