Maybe I am just lucky or not paying a great deal attention to my images (zooming in and looking everywhere) but so far I have been happy. My 75mm f2.8 is really the only lens I had issues with. Mine needed a -10 adjustment, once that was done it got much better.
However I will say every one of my lenses has needed an adjustment but I really don't think that is uncommon. 9 of my 10 canon lenses have adjustments made with the one exception being my 100mm f2.8 IS Macro.
Corner softness on one side is a different story. My guess is that some of these old lenses have been knocked around for many years causing this problem. I talked with Pentax today and they said
Camera Repair Japan | Home maybe able to fix these issues. I believe they are the only people here in the US that repair them.
If the repair costs under $100-200 I would say its worth it if you get lenses for a cheap price.
I really hope Pentax takes the old designs and updates them or start making the old lenses again.