I am normally very skeptical of new products of any kind until they've been around a while. However, Under Armor has definatly passed every test I've thrown at it.
I used it when I worked in maximum security prison with no airconditioning wearing a stab proof vest. I was working in tempratures of 100-120 degrees. It helped wick the sweat away tremendously.
I use it almost every day as an under shirt and it's a 1000 times better than any cotton shirt.
I use it as my gear while I'm training for a marathon and would not be with out it.
I've also used it many times while hunting and I can personally attest that it is not "gimicky" or "the same as spandex". Under Armor is a product that I picked up and used for hunting before they were a hunting product.