I have the K10D and am very happy with it (I am a relative newb to DSLRs, so take my opinion for what it is worth).
I chose it over the Nikon and Canon of similar cost because of the build quality, seals, in body IS, and the layout of the controls. It also has some neat shooting modes (like aperature priority).
I knew I would be worse off as far as fast auto focus, but the new lenses that are rumored to be coming in the near future should do better. In good light I think it is on par with the other brands (of similar price point, of course). It seems to be not as good in low light.
You can see some of my shots here:
Pentax K10D Photo Page
Sea Otter Classic 2007 - Sunday
There doesn't seem to be too many "Pentaxians" on this board. I do see a lot of discussion on some other forums.