I'd like to say that I've owned my Pentax K10D for about 6 weeks now as an upgrade to the Pentax ist DS that I also own and I have shot approx 500 images on it. I'm a keen amateur and shoot a lot of different types of photo. It has so far done some studio work, some macro photgraphy, some wildlife photography some low light photography and a lot of general day out with the family type stuff, so its had a variety of situations to handle.
My impressions of the camera are very good, It is very comfortable to hold, and feels very solid. I have tried a Nikon D80 and my wife owns a EOS 20D and it feels more sturdy that either of them. All the main controls are easily to hand and easy to operate, but the battery card opening mechanism could get annoying if you change your cards a lot during a shoot, although it does mean that the camera is weather sealed to a level that you'd normally only get on pro cameras.
The camera is also easy to understand and work with, the two-dial method of changing shutter speed with one dial and aperture with the other is fantastic ( I had a Pentax film camera with the same method and it was much missed until I got the K10D) and enables you to get the combination you want without your eye leaving the viewfinder. I have used the EOS 20D and, whilst good, it is far less intuitive in its operation and control.
The lack of programmed picture modes may scare the novice, but there is a Green Mode, where everything is set for you, so you can just point and shoot, as well as a number of priority modes, where you choose the shutter speed / aperture / sensitivity level and the camera will work out the rest for you.
The Image quality appears to be up to the usual Pentax high standard. The K10D produces some excellent RAW or DNG format files. Be warned though, Pentax dont tend to go for the highly sharpened and saturated method that other DSLR manufacturers will use, so expect your images to be more natural and subtle than vivid and punchy.
The big letdown for me with the camera is the software that you need to process your RAW / DNG files is badly designed and confusing. Despite the fact that a manual for the software as well as for the camera is supplied, The software is still complicated and uninspiring. Worse still, as I type this (early March 2006) there is little Decent Third Party software support. Adobe Lightroom works with the DNG format, but not the PEF format. There will be no Pixmantec Rawshooter support (bought out by Adobe)and Capture One LE will support it in version 3.7.7 due in the spring.
The list of lens currently available is not huge either, but all the major bases are covered and companies like Sigma and Tamron also support the Pentax Mount. You can also use nearly every previously made Pentax Lens (some may have limitations) and Pentax Mount lenses from the year dot, so you can get some good lenses for little money with a little searching on ebay and the like.
So in summary, if you can put up with the software, The Pentax K10D is a great camera that will suit users through from the beginner through to semi-professional photographer and deliver great results.
I am very happy and can't think of another camera for under about £1500 that I would swap mine for.