I got a KatzEye for my Canon 30D. I used it for maybe a month or so, and ended up kind of frustrated and went back to the original screen. It was a good experience, but I found that it prevented me from enjoying photography. So here's my list of the problems I had (when I was investigating them I found lots of reasons to get them, and very few complaints about them; I assume you've found the same so I won't go into the pluses here):

• The Canon 30D isn't designed to allow the user to change focusing screens. That made things a little tougher (I opted to install the screen myself instead of mailing my camera to KatzEye and having them do it for me); if the Pentax K10D makes it easier to install a focusing screen, you may have a better experience than I did.

• I've never used a film SLR, and hence never used a manual focusing screen. I found that the the extra lines on the focusing screen were distracting.

• The viewfinder was just a little bit darker than the original, which made it difficult to use the bracket-focus method I used to use when manually focusing: I had to use the split prism (it was fun using the prism to line up two edges, but it was sometimes actually more difficult than focusing with the original focus screen since the viewfinder was darker).

• Since I had to use the very center of the frame to focus I ended up taking most of my photos with the subject still in the center of the frame, instead of (more pleasingly) to the side. So taking photos with the manual focusing screen was negatively affecting my composition.

• I opted to install it myself, instead of sending in my camera. I was never able to get it to focus super accurately. In tests (a ruler at a 45 degree angle at MFD and f/1.4) it was close, but not perfect, compared to auto-focus. But in the field, I always ended up just missing focus. Actually, I'm not positive that it was any less accurate than the original focusing screen, but I got used to bracketing focus a little bit at wider apertures with the original focusing screen to ensure that I ended up with good pictures, and I didn't do that as much with the KatzEye, so I ended up with fewer good pictures.

• I got a few specs of dust or scratches or something on the screen (as well as a small blemish on the mirror) when I installed it myself. Whatever they are, they're not visible to the naked eye, but they show up as visible blotches through the viewfinder, which is kind of distracting.

Eventually, all of this (the just-missed focus, the dust, and the extra detail on the screen and my poor composing) got to be too much and I switched back to the original (although it has some dust or something on it, too, now, which is visible through the viewfinder). A lot of the problems were my fault, trying to install it myself, but even if a professional had installed and calibrated it, I think I still would have ended up dissatisfied with the 'noisy' viewfinder and the distraction of needing to center on the subject to focus accurately.