George,
While I have not used this lens on Pentax, this was the lens that stayed on my Canon 350D 90% of the time (before switching to Pentax). Comparing it to the Tamron 18-200 - Sigma was considerably sharper (at least my copy). Very versatile indeed, but as suggested above, little slow... Nevertheless, I enjoyed it very much. Here are few samples:
18mm
21mm
24mm
50mm
200mm
88mm
It indeed covers the range for everyday use, and travels well, but, as suggested above, does not offer results as sharp as 17-70mm DC...
If you want all in one everyday lens that stays on the camera, I found that 28-200 Tamron XR IF works very well, little faster, great colors on Pentax... Yes, I lost 10mm of the wide end, but often, you can find this Tamron lens on Ebay for $100 (or about) band new. At $200+ savings, I find this version of Tamron a great alternative to Sigma's 18-200.
Hope this helps.
Regards,
D