I use mine to shoot girls softball, Little League baseball and soccer. Of course those are all outdoors in the sun, or under the lights, so it isn't really challenging the ISO or lens speed. If I start going indoors for basketball or wrestling then it will get tougher to get what I want.
Every time someone posts that a camera isn't fast enough for a certain type of photography I have to chuckle. I've used Graflex Crown Graphics to capture some pretty good sports photos. I have also gotten some pretty good wildlife shots by setting up a blind and letting the wildlife come to me. Face it, compared to some cameras regularly used by photographers only 50 years ago the Pentax 645D is blazing fast, and a lot like magic.
Not everyone has to have the fastest camera or blazing focus speed to get the photographs they want. Sometimes all it takes is just to change your approach.
Example 1 - The batter just hit the ball out of play. Can you see it?
Example 2 - The batter is preparing to hit the ball that is quickly approaching. Can you see the ball?
These were both taken with fully manual, mechanical cameras with no exposure or focus assistance at all. Compared with these cameras, that I do love using, the 645D is a relative speed demon.