Recommended to me as being ideal for wearing under motorcycling gear all year round.
I read of a chap who rode from Manchester to Chennai, India, on a motorcycle and he wore merino wool base layers the entire way and had nothing but praise for them.
As an all weather rider I had to try them. I've only tried it once so far and was still cozy after about four hours riding on a cold wet day recently.
Light weight, non bulky, packs down small and extremely comfy, it was on sale at half price!
The next test is putting it through the wash, fingers crossed.