Whether it be Goodwill, St. Vincent dePaul, or one of the many for-profit resellers of used goods, thrift stores are fascinating (and fun) places. My wife has found great shoes and purses at thrift stores, good expensive brands that are in like new condition. I've found plenty of books. We've found any number of little decorations for the house. When you find a good thrift store you can find great stuff.
People of all situations seem to use thrift stores - for some it's an essential place to shop for essential clothing, for others it's a fun way to find deals, and hipsters love 'em because these stores seem so anti-establishment to them (and they can get cool t-shirts). I've seen people in thrift stores that look like they're about to head to the opera.
A great slice of life, a great place to people watch, a great place to find deals. If you don't go, you ought to!