What is so special about merino wool, you ask? Well, here is some information from the wiki article I just wrote for you uninitiated:
Merino has several positive traits when compared to other popular outdoor clothing fibers such as cotton or polyester. Merino traits:
*Fibers will warm the body in cold weather and cool in warm weather.
*Fiber size is at the 18 an 19 micron level, which is below the sensitivity of human skin, so no one is allergic to it, unlike regular wool.
*Fibers feel much softer than regular wool due to the smaller fiber size.
*Has natural odor resistant properties
*Wicks body moisture well
*Warms when wet
One disadvantage that merino has is that it does not dry as quickly as polyester or other similar synthetic materials. This can be an advantage in hot temperatures, since the moisture will help cool the body when evaporating, but can be chilling in cold, windy weather due to the moisture’s evaporation.
Merino clothing makes an excellent base layer and/or mid layer during the winter. The positive traits above show up well when the clothing is against the skin or is used as an mid layer. An outer, more wind resistant layer, is recommended in cold, windy weather since most merino weaves are not tight enough to block wind. There are some merino garments available with wind-blocking liners, but most are 100% pure merino.
In hot summer weather, the lighter merino fabrics (140-150 g/m^2) do well in regulating body temperatures, handling moisture, and resisting odor. The heavier weights are not recommended since they can cause you to retain too much body heat due to their increased density and often tighter weaves.