Merino wool: isn't itchy and you can wear it for days without it smelling foul...together with the other fine attributes of wool clothing.
This review goes also for the pants as well.
Got these initially for cold-weather motorcycle touring. Have used them in cold, rain, and even in warm climates when you don't want to stick to your sleeping bag or whatever due to the sweat. Hiking in Switzerland last summer, I was wearing a Lowe technical shirt that zipped up at the neck that was supposed to keep me warm and dry. I was wet with sweat and thus, quite cold with the wind. I put up with it for a while and then thought, "Why? Because it's a new shirt?!" I took out my Icebreaker shirt (same holds for the leggings), took off the Lowe shirt, changed and was kept warm the rest of the trip. I took the Lowe shirt back to the store where I had bought it and got them to replace it with another Icebreaker.
Agree with the previous review, warm and not itchy at all. Plus (and I think that's a BIG plus): by far not as smelly as the polypropylene/polyester variety (no smell problem after 4 days of Ski-touring). So price really is the only drawback, but I think it's worth it.
I agree that this is a comfortable, warm non-smelly first layer. However, if you are on the slim side, you may find the sleves too short. For staying warm, it is very important that the shirt covers the wrist. For this reason, I still prefer my old DuoFold. Unfortunately, I don't know the model of the DuoFold I like, and a newly bought one with zip-neck (variterm I think) has a terrible fit - the arms are long enough but much too tight.
Most comfortable lightweight item I have ever worn. Puts my older Patagonia items to shame. And I used to swear by them. Very light soft and wicks well. I have not had it long so the jurys out on how long they last. Very pleased so far.