This is not the first time in this forum people have critically discussed Pentax AF.
There is usually a pattern to these discussions - the discussion will range from: 'people shot action OK even before AF existed' through to ' buy another brand - problem solved'.
I fall towards the side of the discussion that says 'Pentax AF can do the job under most circumstances', even with dynamic subjects that make football players look as static as garden gnomes. Not automagically all the time, but it can be done if the light is good and some experience is applied.
Most recently, I've been able to directly compare the K-3 w Sigma 70-200 HSM II with a Nikon D610 w. Nikon 70-200 VR shooting very dynamic and unpredictable low-light rodeo action. Like, thousands of shots.
To be honest, they both work just as well as each other. The only major difference is the noise levels and IQ in high-ISO post-processing (advantage Nikon FF) , and the slight edge to the K-3 when the ambient light completely drops to almost black. The key to getting good focus results with either in AF-C has been (a) the AF motor performance of the lens, (b) choosing the right AF settings for the type of action and scene, (c) setting the right shutter speed, and (d) anticipating the action/ locating yourself in the right place.
Pix generally related, following the same subject (rodeo), ranging from K-x w. manual focus lenses to Nikon D610 with fast AF lenses.
Explosive entrance - Rodeo at Lismore Show 6/14
Pentax K-x, SMC Pentax M 75-150 f4, 1/2500, ISO 800
Bull riding - Rodeo at Lismore Show 12/14
Pentax K-x, SMC Pentax M 75-150 f4, 1/1250, ISO 800
Kyogle Bull-riding Spectacular - runaway
Pentax K-5, Sigma 70-200 f2.8 HSM II, ƒ/2.8, 83.0 mm, 1/500, ISO 25600
Yellow chaps 1 - Kyogle bull riding
Pentax K-3, Sigma 70-200 f2.8 HSM II, ƒ/3.5, 93.0 mm, 1/500, ISO 1600
Brown bull again
Pentax K-3, Sigma 70-200 f2.8 HSM II, ƒ/2.8, 70.0 mm, 1/400, ISO 6400
Red shirt struggling - Lismore Show Rodeo
Nikon D610, Nikon 70-200 f2.8 VRI, ƒ/2.8, 110.0 mm, 1/400, ISO 5000
Hereford sideways - Murwillumbah Show Rodeo
Nikon D610, Nikon 70-200 f2.8 VRI,ƒ/2.8, 160.0 mm, 1/400, ISO 20000