How curious - two years ago I had the difficult choice of Sigma's 17-70 or 18-200 to take backpacking with my Sony A200. Now several bodies and one brand switch later, I've won back both these lenses in PK mount! I knew I liked them before, guess this proves it.
A curious thing about Pentax is that f/5.0 simply isn't an option in 'Av' mode; as I zoom at max. aperture with most lenses, they jump from f/4.5 to 5.6 at some point. Now I learn that they also don't "do" f/6.3 as above 175mm this becomes f/6.7 according to the camera. More tests are needed to see how this lens does, but it's great having a 400g lens that can do so much! Its closest focus is .30x, bokeh looks fine at first blush.. sharpness tests still to come.