Although the 17-70 is a stronger offering in every way - build, speed, and range - the light weight of the Pentax 18-55 makes it a fine knockaround option. Its weather resistance is a good thing too, even on a less-sealed body like the k-x... but is its image quality too great a compromise?
It appears not - these two shots were taken consecutively at iso320 and Av mode set wide open. The DA18-55 is on the left here, and while a decent bit slower at f/5.6 (48mm, where the Sigma is at f/4) its image quality on this scene is more than adequate. Sharpness looks as good, bokeh a bit better. The Sigma image is set at -2/3EV to match up the exposures, so the DA lens picked a better setting. At around half the weight, and given the k-x talent at higher ISO settings, the result is quite impressive for a lens about 1/3 the cost (actual costs: $95 new vs. $250 used!).
Nice to know my camera has a good all-purpose lens for easy snaps. I still expect the 17-70 to be the choice when image quality is most important though - but both are quite versatile!