This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Does not wick very well. Can be uncomfortable. Troublesome to care for
(e.g. can pill badly) Will keep you fairly warm if soaked. Not very wind
resistant. Shrinks under heat from dryers. Thermax is an improvement on
Polypropylene. The big advantage is that Thermax isheat resistance so you
can put it in the dryer. Balance that against the extra cost.