I did the January 2018 Single In Challenge with my new KP and DA 55-300 PLM. By the end of the month, the KP and DA55-300 PLM had supplanted my K-1 and DA*60-250 for my upcoming Galapagos (and other places) trip. The high IQ and focusing speed of the KP and DA55-300 PLM, combined with their much lighter weight, made the choice easy. My K-3 will be the backup/2nd active camera, and will usually be sporting the DA16-85. The matching of sensor sizes between these 2 was a bonus, making final lens selection easier.
The large grip for the KP is a necessity for me, and didn't take too long to get used to (still not the rock-solid feel of the K-3, however). Battery life is shorter, as many have noted, so more, but lighter and faster charging, batteries will be taken. And I have the D-BG7 battery grip to use the larger D-Li90 battery if necessary. I've always liked the top LCD of the K-5 and K-3, but maybe the smaller top LCD of the K-1 got me used to not so much information available on top. I don't miss a top LCD on the KP at all. Buffer size and burst speed is lower than the K-3, but I rarely hit the notorious buffer limit on the K-1, so not a concern for me.
As most KP owners, with no stores within a zillion miles having one, I bought my KP on Pentax faith. And am happy I got over desperately waiting for the K-3 "flagship" replacement.