Just got a K-3iii, had only the opportunity to try it briefly around the house. I'd like some general impressions and a question and opening a new thread seemed overkill so forgive me for replying to this one on the same topic.
What I liked immediately:
1) The new shutter button mechanism: fantastic, feels incredibly premium and smooth, greatly facilitates steady handheld shooting for me. The shutter and mirror mechanism is also very well dampened.
2) IBIS is a
vast improvement over the original K-3. I think it is not stressed enough in reviews. Managed several 1/20 handheld shots at 100mm with no problems, I even went down to 1/8 with decent keeper rate. Will greatly reduce the need for high-iso indoors for static subjects. It's probably not at the level of the supposedly magic Olympus IBIS+OIS, but it's certainly up to 2022 standards.
4) AF joystick. No comment needed

5) An AF module that finally works. Keeper rate vastly improved in difficult conditions. The off-center AF points are decently accurate whereas they where virtually unusable on my K-3 (unless in perfect conditions with lots of light and small apertures). It breathes new life in old screw-driven lenses that don't do that "micro-hunting" anymore and are perfectly viable (still, I much prefer DC and PLM...).
6) Top notch viewfinder with customizable overlays and lots of info, clear and bright. I admit that I was somewhat expecting more. Perhaps the K-3 OVF was "good enough" for me. I mean, it's nice to look at, but for instance I don't find manual focus any easier.
Not convinced:
1) High-iso rendering. I always shoot raw and use best available NR software (DxO DeepPrime) in my experience this kind of NR engine does not couple well with cameras that apply aggressive preprocessing to the RAW files (they all do to some extent, but Pentax is very aggressive). Or at least, it's very hit-and-miss. The in-camera algorithms for certain subjects, especially if the lens is not particularly sharp, tends to smear way more detail than it should. Whereas some shots with different subjects, lens, lighting even at very high-iso are perfectly fine and come out well from DxO. I have done some ISO 12800 test shots that are great and some terrible ISO 3200, which I used with no problem on the K-3. Will need to make a comparison in controlled conditions.
There is one thing that bugs me: i notice that the bottom/rear of the camera tends to become moderately warm, is it normal ? I am even use it with the back LCD permanently off unless for picture review.
I just did some shooting, no long burst, no video. It just seems to run hotter than my K-3 with which I have
never had that feeling of the camera warming up.