I've been using sock liners for 20+ years. I use polypro in warm weather, and Smart Wool liners in winter. The Smart Wool are a bit thicker than the 'regular' polypro, and but more importantly, add about 10-15 degrees of warmth to an insulated boot. That's way too much for summer wear - at least for me - but essential in winter (I live in northern New England).
Nearly all my hiking socks are Smart Wool (there's an LL Bean outlet nearby which sells Smart Wool seconds for 50% off). Darn Tough makes excellent socks, and my son gives them to me regularly as gifts, but ... they have too much elastic in the cuffs for my taste so I rarely wear them either for hiking or everyday use.