I took it out this evening for a walk around in lower light situations and it performed pretty well. A couple shots weren't well focused straight off, and it will just stay there, but if I aim the AF at something different then back to the subject, it found focus. I'll keep playing with it and seeing what I can get out of it. I guess the world's supply of Pentax F 28's probably isn't changing that fast, if I decide I want to try something else. That's probably the lens I would try next, but I really like having something faster than f2.8.
I did take this with focusing via the AF system. Taken at f1.8. Really smooth out of focus, a bit of green and purple fringing;