I've got the coldgear, duofold, mountain hardwear as well as some natural products. I really like the synthetics as a base layer and believe that you should buy what you can afford. I really like the coldgear, but also really like the duofold. The mountain hardwear is nice, but even more expensive than the under armour. If I hadn't received it as a gift, I probably wouldn't have bought it. The only natural product that I like is Merino wool. It works as well as the synthetics and is a natural product that may appeal to some people. In this form, I can recommend the undergarments from Filson and Icebreaker. The biggest issue that I have with the underarmour is the fact that when you put it on you look like a stuffed sausage. In my case I look like an overstuffed sausage...a way overstuffed sausage.
