Have tried several bib tights (Castelli, Louis Garneau) and the Pearl Izumi's are my favorite! I've ridden in the mid 30's F and for me, I wouldn't wear these in any lower temps by themselves. On the upper end, they've been comfy up to 60 F. I also like that they don't have ankle zippers, which make them more comfortable with high top boots/shoes. I'm 5'7"/160 lbs and the medium fits just fine. My one complaint is the flimsy shoulder straps, they tend to fold over like a shoe lace, so they could be a little beefier.
These tights are great; I'm comfortable at 40F rides (haven't tried much colder) and 50F is my upper limit. They don't feel restrictive and I was unsure about the padding at first but now it's great.