Yes, it's heavy and expensive for a baselayer. But you get a merino layer that you can live in for weeks, and that actually saves weight and bulk on multiday trips. Our tester used it in the Rockies near Colorado's Crested Butte, with temps in the 30s, and reports, "I wore it alone, under a shell, and in my sleeping bag. It's so warm–and so soft next to skin–that I left behind a bulky fleece and a lighter-weight baselayer."
The remarkable softness is the result of two manufacturing processes: The wool is slow-washed (without bleach) to minimize itchiness, and the interior is terry-looped (like the inside of premium hiking socks) for extra softness. A 9.5-inch-long zipper enhances venting. The cut is long (four inches past the waistline on our 6' 1" tester), sealing out hem-level drafts, and wide enough for broad shoulders. $150; 12.5 oz. (men's large); men's S-XL, women's XS-XL; patagonia.com