I had an *ist D and then a DS and then a K100D and have used the K10D for quite a number of times before I finally "switched" to the Canon 5D.
In terms of digital image quality, I must say the K100D is the best amongst all Pentax DSLRs, say, accurate/natural colours, good DR, well defined jpegs etc.
The *ist D is weakest for the digital department but it has some better inside qualities, the shutter and mirror action is smoothest amongst and vibration is minimal. The sound (and thus the moving/controlling mechanism) is somekind of MZ-S alike but it is a slower version :-( The traditional TTL auto flash of the *ist D is more accurate than that of the DS too (which tends to overexpose)
The worse mirror action belongs to the DS (it vibrates vigorously and the sound is loud) which I guess Pentax did have sucked too much quality from that part of the design.
If you have the luck, there are very early version of the *ist D which has more rigid construction and had slightly better exposure accuracy. To check, just press the RTF cover from the top and there is no play of any kind at all and the plastic did look blacker. My wild guess is that the very early batch was made in Japan and after that that batch all *ist D are just more filmsy, even for what the paintings looked like.
Originally Posted by carpents
I had the *ist DS first, then the K10D, and finally the *ist D. The jump from *ist DS to the K10D was a big one from an added functionality standpoint; however, compared to the *ist D the K10D was more a jump in ergonomics and usability. Virtually all of your (and my) complaints about the *ist D are covered in the K10D. I can fault the K10D for only three things missing from its younger counterparts - erratic metering with old lenses, increased noise at high ISOs, and no TTL flash metering.
IMO the grip is better. The camera is WAY faster at everything. The 4-way is much better (not that hard to do!). The scroll wheels are in (subtly) better places. The ISO is changeable on-the-fly (via the OK button and scroll wheel) as well as being selected in 1/3 stop increments. Exposure compensation can be assigned to one of the wheels. The menus are much more intuitive and useful, also easier to get to.
But in all, they are all good cameras. I have vertical grips for the K10D and *ist D and IMO both are better with them. If you have bigger hands IMO they are a must.