There are a
few folks on these forums that have valid criticisms of Pentax AF.C performance. Many, I would hazard a guess, criticise it too quickly in order to compensate for a lack of skill. Not all, but many. I say that with some personal experience, as I struggled with it too, and still do on occasion (despite being a reasonably competent photographer). But the fact is, skilful use of the camera can often overcome the limitations. I'm probably the forum's worst "birds in flight" photographer, but when I've followed the examples and advice of others here that are more experienced in this discipline, I've managed to get some very decent shots. That advice, incidentally, has helped me on more than one system - not just Pentax.
As for AF.S performance, I have absolutely no problems. Pentax appears to compare very well with other DSLR systems using phase detect AF sensors, and once AF fine adjustment is carried out for the relevant lens(es), accuracy is generally excellent