I haven't tried them yet but I have a giant bag of inexpensive clothing on my dining table to send them. I've heard they give very little money for your items but they do take certain brands that other companies don't, such as Old Navy. When I recently lost almost 40lbs I was constantly buying Old Navy jeans every few weeks from going down in size so I have quite a few pair of almost new jeans to get rid of along with other clothing items of "lesser" brand names.
I did use Twice (liketwice.com) to sell most of my other clothing that was too big and was very pleased with them! They emailed me their offer within 5 business days and I was happy with the value they assigned for my pieces. Twice does not take brands like Old Navy, however (they do accept Loft, Gap, etc, though).