In reply to ow arm:
My problem with merino is its fragility. I have loads of synthetic base layers (some very ancient), and they have snags, pulls, road rash holes, etc, but I've never once had to apply needle and thread to prevent further damage.
My MAPP merino items have just about all developed holes that very quickly run, so that I've had to darn them all. I'm pretty sure that the saying "a stitch in time saves nine" originally applied to knitted garments.
Which is a shame, because I love the low-odour qualities, compared with PP and PET. Given how expensive the stuff is, and how fragile I find it, I don't think I'll be buying any more.