Just got the chance to use this last weekend where temps dipped into the low 30's to high 20's. With just a base layer and this guy on, it seemed to block light wind and keep my warm down to around 40 degrees. It backs tiny and clips to your harness, perfect for shady belays! At night when the temps got really cold, I layered this over my Marmot Vars fleece. The combination of the two thin layers was enough to keep my toasty warm in the temps stated above. Impressive considering how this and light this piece is. I'd like to point out that this does not have the drawstring hem that's advertised on the site. The hem at the waist is loose like a tshirt with no adjustment. Secondly, I'd disagree with slim fit. I'm 6'2" 160lbs and while the arms and shoulders fit well in the large size, the waist area is quite baggy. Overall a great piece that has it's place and I'm happy with my purchase!