Sorry Peter,
Welcome to my world.
Now that such a well respected moderator and photographer has said it, I know I won't be mercilessly flamed.
I have been loyal to Pentax for decades. I still have my first Pentax camera, the MX. The QC problems I had with the 16-50 requiring me to go through 3 copies before I just got a complete refund, started to sour me. Then, about a day before my 50-135 was to go out of warranty, the SDM failed. Then I got a replacement from Pentax, and soon after, that one started showing the telltale signs of "SDM". So I got that one replaced, and then promptly sold it. The QC problems, along with the glacially slow AF told me enough was enough.
Those experiences made me do something I hadn't before: try the competition. I spent some time with a 7D and a lens that was ring-USM. I was dumbfounded. It was like using AF for the first time. It made me a bit angry that I was ignorant of how far behind my brand was. The Pentax forums rumors about how the competition sacrifices accuracy for AF speed were just wishful thinking from people that didn't want to face the truth. I tested in every condition I could think of; dark rooms, light, outdoors, sodium lights with motion towards the camera. etc. etc the number of shots I took that were out of focus were very minimal.
Again, I completely understand your frustration, and I share it.