I've been using QD for several years now. I first became aware of them through their military branch lowers, but the service and AR parts selection keeps me coming back.
Quentin Defense manufactures their own receivers, as well as receivers branded by other companies. The QD billet receivers are on par with the best in the business, and I've built several rifles based on them in both 5.56 and 7.62.
I also use QD for FFL transfers. Their prices are good, and they minimize any hassle. All-in-all, Quentin Defense is my preferred gun shop in the valley.