Still a bit too early, but I get a lot fewer false positives in apps like Procreate. I used to get a tool change often when I would move the pencil slightly in my hand, now it seems to take a more deliberate double tap, which is great.
I'm able to do it, but it is a very awkward feeling. You use the index finger to stabilize the pencil, so lifting it off to tap makes the pencil much more susceptible to shifting around between your thumb and middle-finger. I've kind of gotten used to it though.
Good news on reliability of palm rejection, etc.. though. I can't wait to see how developers use PencilKit in their apps!