I've bought a few things from them in the past and everything went very smoothly. They typically have pretty good sales and I've heard their red dots are hard to beat for the price.
Agreed with all above, super fast shipping... great deals on a lot of stuff when they run the limited time offer sales. Little Jack's has been able to hook me up with the same "deal" when I needed a few sets of Troy sights though, and he's local .
never had faster shipping than Primary arms.. bought a picatanny rail riser for a gas block that was sitting too low and it ARRIVED at my house on the third day after I bought it.
I've also had good experiences with Primary Arms. I bought p-mags from them and I also bought one of their red dots. Service and shipping is phenomenal. And their red-dot is probably the best in its price range. Mine hasn't lost zero and the reticle is crisp.