In reply to David Cowley:
> my Patagonia merino wool long John's have developed a whole in the crotch big enough to crawl through
If they still stay up and you're not going without undercrackers there's still life left in them.
My Icebreaker 200g leggings (which have done the same thing, crotch-wise) were adequate for winter belays. My legs don't get too cold, though, and I've tended to put more favour in not-too-sweaty rather than toasty-warm.
My replacement pair are a merino-synthetic blend, in the hope they'll be a little sturdier.