Warm and Extremely Comfortable
I just took 2 of these baselayers on a 10 day trip through the San Juan Mtns, Arches, and Yellowstone. They may appear thin, but they provide a very large amount of warmth. I layered these under a 100 weight fleece and was fine in the 40's. They are definately worth purchasing (much cheaper than Under Armor). This site also provided its impeccable shipping speed.