Originally posted by eyrr The K50 and KS1 are not
That's half-right. The K-50 is not compatible with KAF4, but the K-S1 is with a firmware upgrade, same as the K-S2.
Oddly, the K-S1 is still listed as a current product on the Pentax Japan website a couple months after it was discontinued in the US; the K-50 was listed as discontinued in Japan weeks before it was announced in the US.
Of the three, the K-S1 is probably closest in size, weight, and feature set to the K-m, but its odd ergonomics (and that limited feature set) are quite off-putting to many. The K-50 was affected by a large number of aperture block failures (about one in four was the last I heard for the PF survey results). The problem may have been corrected in later batches; only time will tell, but for now, I can't recommend it, even though it is otherwise an excellent camera. Buying new, I'd recommend the K-S2. The size is fairly close to the K-m; it is heavier, but almost any new body is.