It keeps me perfectly warm, whether I wear a t-shirt underneath or not. Don't need any other sweaters, unless it gets really cold.I ordered a Small, which is pretty snug, and I'm 5'7" at 135lbs, so make sure you get the correct size. If you get a size too small, it probably won't fit.Also, the hood is not one of those loose, flappy ones. It's meant as a base layer, so it sits tight around your head. Great for warmth, but it makes you look like you're getting ready for a deep sea dive.The wool does exactly what they claim it does, washes easily, dries quickly, doesn't retain smells (much). Weighs just a little bit more than a t-shirt.