I went to one in Richmond, VA that was pretty lousy, broken glass in the clothing bins and whatnot. However, There is one about 4 miles from where I am, and it is actually pretty fantastic. You have to go there with an open mind, and you may not find that one big score, but it is an easy spend $20-$50 and make $200+ every time I go. I found a couple 30's GE fans for $3 and $5, some super awesome vintage jansport frame backpacks, a 48 star american flag, tons of high dollar vintage clothing, hats, and shoes, and tons of "spend $1-5 and make $20-$40" items. Its not glamorous at all, it can get a little gross at times, but it's good money if you know what to look for. I think this place is a rose among thorns because it processes overflow donations. There are literally huge boxes of raw, untouched, never before searched donations getting pulled out of trucks with a forklift and being dumped directly in the back lot every 15 minutes for us to look through. There are a bunch of Haitians that hang out there all day and cherry pick, but they seem to buy athletic shoes and handbags pretty exclusively, so it is quite easy to find things that they overlook.
There's another handful of guys who basically live there, a couple reside in the book bins, another guy only buys sports equipment. They're honest guys like us though, and very "if you scratch my back I'll scratch yours". I gave the sporting goods guy an old hockey helmet once, and he gave me anything he found that looked antique-ish (some pretty good stuff), I gave the book guys a garbage bag of CDs I found in a box and they gave me a huge pile of hats from one of the boxes near them. Its actually quite nice, there isn't much toe-stepping and everyone is pretty friendly. The Haitians get a little worked up when a new shoe box gets dropped and the elbows can start to fly, but I just step away and let the feeding frenzy go.
I once went there when I had some spare cash and spent $60 on two huge duffel bags full of merch. I got a bunch of vintage stuff that would be very hard to sell online because it looked cool but was not name brand and took it to a local and pretty hipstery consignment place that pays cash on the spot. I sold them about 1/3 of my haul for a cool $108. Best of all, I had saved most of the better stuff, like a brand new pair of L.L. bean Boots, to sell myself on ebay. I've probably sold another $50 in random jeans and stuff from that haul, and I still have most of it left. So yea, if you can go in with no expectations, it can be great. If you go in hoping for a gold mine, you'll probably be disappointed.