In reply to captain paranoia:
'I suspect that the reason Merino seems to provide a 'drier environment' is because the fibre itself absorbs water, about 1/3 of its weight. Synthetics (esp. polyester) absorb almost no water. So, in moments of peak sweating, the wool absorbs water, giving off heat. In later, less active periods, this water is then evaporated from the fibre. Both merino and synthetics will absorb water into the structure of the fabric by capillary action, but the additional absorption in merino provides a greater 'sink' capacity than synthetics, so is able to cope better with peak demand. Possibly. '
That seems to be what I was struggling to say but with more detail and greater clarity. Ta.
There is no doubt in my mind that the reason people moved away from wool was because of issues with skin irritation, the fact that wooly jumpers became much heavier when wet and that care was an issue.
The quality merino wool now available will not irritate your skin (unless you have a genuine allergic reaction to wool) and is very easy-care (you just chuck it in a warm wash). As long as you wear a thin thermal, the weight gain is insignificant.
There is still the issue that certain very sweaty types seem better off in high quality synthetic, and I'm not sure why that should be (myself and Woody wear the same outer layers, differing only in our thermal preference. He is much sweatier than me tho).
There is no question that I stay drier (and hence more comfortable) whilst wearing merino thermals (and socks).
One problem with merino is that its quite pricey (though Aldi seem to be remedying that temporarily). But I've only got one set of merino thermals and that seems to be enough...
Please also note that Rock+Run, who I 'work' for don't sell merino thermals so I'm not trying some obscure promotion here. (to be fair we'll have a small selection come October but that's a long way off). I think that most folk would be better off in merino though.
Kirkpatrick did a good article in High not so long ago about merino wool.