My legs get cold often when resort skiing - particularly sitting on the chairlift and whatnot. I generally wear Hot Chillies on my legs but, before spring days, they aren't warm enough for me and my legs get cold. I wear just a gore-tex outer shell on my legs - not insulated ski pants.
The R1 fleece pants are super warm, super comfy and keep my ass and legs warm when they need to be. I think the coldest temp I've worn them is maybe in the low teens/high single digits - they kept me very comfortable. I have also worn them when the temps get into the upper 20's low 30's and I haven't felt at all uncomfortably hot either.
Basically, in my experience these pants are a versatile base layer for days where the temp is down to single digits or up into the 30's. Again, my legs get cold pretty easily so pushing it into temps the 30's might be a stretch for most (unless you dig on sweaty crotch!) but I'm comfortable in them. These are my everyday base layer under my ski pants at this point; I like them a lot.
As for sizing, I think they might run a tad big. I'm 5'10" 170 lbs and the large is slightly bigger than I'd prefer, although they do fit without falling down. I'll buy the medium next time.