What I liked about my three Sigmas, and why they could be expendable.
The 17-70 is a great do-it-all lens: very good focal range, decently bright, good sharpness and bokeh, and extremely good at close focus. I have already let this go though.. what happened? Well, it's a bit heavier on the k-x than I liked (I am an admitted weight weenie), and the 16-45 is cramped in focal length but goes a smidge wider and focuses nearly as close. I perfer the color and contrast a bit more on the DA lens, and the quick shift is something I missed. And this is one of those Sigmas that zoom in the other direction relative to most other models; of course the DA16-45 extends the opposite of others' so this is a relatively small knock. Other than not going out to 70mm, the DA16-45 is just more matched to my needs, it seems.
The 18-200 is another reverse-spin zoom, but boy does it have range. Think of it as an 18-170 f/3.5-5.6 and you'll see why the kit zoom moved on despite the quick-shift and light weight. My 18-200 weighs in under 400g and is good with sharpness and bokeh. Its downfall is that Pentax reports all over 170mm as f/6.7, even worse than the stated 6.3 (splitting hairs here though). It may stick around as the ideal quick-snap option though, the one lens to carry when you are not out to do imaging but just capturing opportunities. It could use a bit of PP to really sharpen up relative to the Pentax lenses I have (16-45, 50-200wr). And when focusing close with the 18-200 (which it does well) the effective FL up close is more like 170 than 200 fully extended. All hi-range zooms do this though, one of the 'compromises' that these lenses inflict on users. We'll see if it stays around..
The 18-50 f/2.8-4.5 is a new and curious lens. The price is quite nice, it does not change size while zooming or focusing, focus is nearly silent, and it's mighty fast. It's only f/4.5 from about 45-50mm so it isn't one to go faint quickly, another positive. It also has stabilized optics, which Pentax users don't need but my tests suggest it does a slightly better job than SR in low light. As an indoor lens it would beat out the 16-45 for silence and brightness is used wide open.. but that isn't a common use for me. Its close-focus talents are not close to the 16-45, and its range is more cramped. Sounde like this one will not be staying.