In reply to tomcf:
Well, this thread just goes to show there is never one answer for everybody.
I've got several wool baselayers and think they're OK, but maybe overrated. I find they don't wick as well as new polypro/Capilene and definitely take longer to dry than any synth. But of course this depends on your exertion level and then how much sweat, or not.
On the plus side they don't get smelly like synths - so good for long xpds or travel - and they are more hardwearing. I've used one Icebreaker 260 weight top a lot for 2yrs and it looks like new. My polyrpos and similar look ratty after 6mnths or less of real use. Irritation to skin, or lack thereof, will always vary with people, but most of the modern garments are fine for most people.
BTW - 'wool' in this sense is a wool/lycra mix as used in all these modern garments. NOT 100% wool, as that does not wick as well, can be scratchy and takes longer to dry.
D