I come from the camp of 'had a Pentax already, so why change now'! The K20D is my 3rd Pentax DSLR, having previously owned an *ist DS (which was passed on to a newcomer to SLR photography), and with a K10D currently sharing it's stable.
I'm not vastly experienced in all makes and models of camera, in fact I've hardly used anything other than Pentax, so my review is not comparative and therefore perhaps 'slightly' skewed. I'll try, however, to remain objective and honest about my experiences.
Although I've been taking photos for nearly 15 years, I'm still learning a great deal, as my previous cameras had 'picture' modes which I must confess suited my fear (and a bit of laziness) towards setting shutter speeds or apertures manually. No doubt some readers will be disgusted and not read any further!
The K20D forced me to become more involved with the camera, and I've learned more about using a camera (and it's potential) in the past 12 months than I did in the previous 14 years!
I've been consistently overjoyed, on reading articles about photography methods and techniques, to discover that not only is pretty much everything/anything possible on the K20D (and the K10 I might add) but it's also incredibly simple and easy to achieve. Yes, there's the odd consultation with the instructions, but generally it's been so quick and easy to expand my abilities that I've sometimes wondered if I'm missing something vitally important!
I know the review on this site says the menus are 'dated', and I'm not sure exactly what that means (is it a 'veneer' problem, or an issue with usability?), but I've found the camera extremely easy to set up to my own preferences, and the menus I find to be self explanatory. Maybe other systems are quicker/easier to set up? I certainly can't complain.
I use several Sigma EX-range lenses, which I must say add a level of quality over 'kit' lenses that is amazing, and at a price that is much more reasonable than their Pentax counterparts.
My only quibble with the K20D (and I marked it down on performance as a result) is with the autofocus. On the whole it performs very well, but there have been one or two occassions (that I didn't consider to be overly taxing) when it struggled, and on two of those occassions I lost the 'moment' as a result. I'm still trying to emulate and/or discover what will confuse the AF system, so I can prepare for future shoots with those factors in mind, but it now refuses to give anything other than a perfect AF every time so I'm none the wiser!
And so, if you'll forgive my extended ramblings, I'm very pleased with the K20D, and I was amazed by how quickly I became comfortable and confident with it in my hands. It's quick and easy to change key shooting modes, it's robust build inspires confidence, and I think both it and the K10 will be my photographic companions for some considerable time to come.