I used these for a recent spring skiing trip with shells over them and was comfortable all day.  Did not get too hot and they were breathable.  I've worn them under pants in freezing weather and have been toasty.  The knees get stretched out so even though they are thick enough to wear alone, they look goofy so don't buy for this purpose.  I have a pair of Stoic brand midweight bottoms and can compare.  They are definitely softer, almost cotton like but the Stoic ones don't stretch out--must have some lycra in them.  These also have a "mom jeans" cut to them so be aware.  All in all the coziness beats out all the cons and I pretty much wear them around the house as much as I can.  Do wash inside out as they do pill on the first washing only.  Sizing:  I am a size 12, 5'9" with a fuller bottom and ordered the large.  They fit at first but do stretch out as I said, probably could have gotten away with the medium and then maybe the knee thing wouldn't be a problem.  It's hard to see but they have a cuff at the bottom typical of longjohns