I chose Pentax for the small system so after comparing this lens with the Sigma 105mm EX MACRO I decided to go with the Pentax. Very small and light (345 grams) compared to other marques. Tack sharp from f/4 - f/11 and still very good wide open at f/2.8.Virtually no distortion, vignetting or chromatic abberations. Excellent colour, very neutral and bokeh is creamy smooth.
Clamp locks focus to allow 'Back and forth' manual focusing technique. AF is slow and noisy, this is par for the course with macro lenses but the lack of a focus limiter is unforgivable as it results in severe hunting. Not as well built as the considerably heavier and larger Sigma 105mm MACRO. One of my favourite lenses with clear and vibrant results I can rely on.
I agree with what you said about this lens. The lack of focus limiter is really too bad. That said, even if the focus throw is very long, it's very fast at least on my K10D/K20D. The only annoying thing is that something it goes in the wrong direction and that slow down the AF considerably.
I have this and I like it too, but from a more limited experience of Pentax lens
Do you have the 16-50mm F2.8 ED AL DA* - too expensive for me to just buy it hoping its good!