I have used my Icebrakers now of several multi-day backpacking trips and I can say for sure they are worth every dime. The merino wool doesnt hold stink after several days like synthetics do and is very soft, not itchy at all. I wasnt sure about how well they hold warmth but can say i was toasty down into the teens on one trip. Some products are worth all the hype...this is one.
This is hands down the best (and most expensive) long underwear you will buy, if these cant keep you warm nothing will. I wear them all winter under ski pants, hiking/climbing pants, and even under jeans around town when the temps drop. Sizing is tricky though, they run a little big and you want a snug fit here, go a size down, they should stretch and fit.