I think the most successful camera for Pentax image was the K10D, it was truly an award winning ground breaking dSLR for the performance and features it had. It brought available features such as WR that were once only available to Pros with budgets for $6000 dSLRs to the masses. This established the benchmark for all other Pentax dSLRs to follow.
The K20D IMHO and probably biased because I own it. Is of course an upgraded K10D, but this is a great thing because the K10D did many things right and some better than any, speaking of ergonimcs, feel, everything. Some problems such as high ISO noise, and some needed/wanted features such as PcSync port, AF fine tuning, improved JPEG engine. The K20D gave all the things we wanted plus a new Samsung 14.6Mp CMOS sensor that had the highest resolution to noise ratio of any APS dSLR at the time of launch. We found out that truth over one year latter when DPR tested its RAW output against the D300. In any case it was and still is a very solid dSLR, problem free. Problem free because its apparent Pentax watched and listened to every legitimate complaint against the K10D and addressed it. Even the poor SD card reader mounting was addressed something we never saw. AF fine tuning, two different JPEG sharpness settings, all from us and reviews.
The K-7 was a completely different camera and it has perhaps the best body, build, feature package versus any APS dSLR at the time of it release date. Smaller metal shell, very tough build with improved seals to withstand colder temps. This alone should say what Pentax is about. They are not built to be the best on sports sidelines, but be the best climbing a mountain during a snowstorm, or walking down a dusty road, or hiking and it starts pouring rain harder than you have ever seen. I could write much more but...
Was the K-7 a success, honestly not as much as I had hoped. It was a better camera, it was a smaller body, more features than any before. Pentax said before its release it would not replace the K20D but be a whole different dSLR higher performing of course. And Pentax Japan still has the K20D listed as one of its main products. Then comes the K-x which had the high ISO we thought the K-7 would have. Now I don't shoot much above ISO400 so... but many bought the K-x for its clean or usable ISO6400. I believe the Samsung 4 channel (the K20D was a two channel) CMOS sensor let Pentax down, I bet Pentax was hopping for much more performance than it got.
No one is right or wrong, this is just IMO, how I feel, true or not. I feel the K-7 concept was a success, its body and features were ground breaking almost as much as the K10Ds, it was a superbly designed body and dSLR; the best in that catagory for sure even today. Pentax can build bodies-dSLRs second to none and I really believe that. But I feel it has much more potential, becuase its performance was no better than any of its peers and in many cases not as good (say a D300s) it just needs a few upgrades and it would be among the finest APS dSLRs ever made, or overall the finest! And this is from a die-hard Pentax Fanboy...