User Review: Icebreaker Men's Rock Zip
Fabric: merino wool
Excellent light sweater/midlayer.
I am a believer in wool when it is really cold. It's not as light as synthetics, and it doesn't wick quite as well, but when it is really, really cold, wool feels warm to me and synthetics don't (this is especially true for base layers).
I have used this piece a lot in the Adirondacks for winter mountaineering in combination with a light baselayer (either synthetic or wool, depending on the temp); that combination carries me most of the way up to tree-line on single digit days (maybe starting off with a softshell jacket or vest that reappears at tree line). It is an excellent piece, and well sized.
One gripe: the cost. Thank goodness for EMS's panicky approach to clearing out stock. As good as it is and as versatile as it has been, I wouldn't pay the price you usually see for it.