The K20 has been around for more than 2.5 years. Lots of folks use and shoot the K20. There has not been any reported systemic problems with the K20 during this time. So far it appears to be a very reliable, consistent and stable performing model.
I acquired mine a bit over 18 months ago just as the K7 was announced and the K20 pretty much hit its price low (for a new unit). I have been extremely happy with mine. That said, its back at CRIS for a second warranty repair. I shoot after work - late afternoon, dusk and evening - basically, its dark. I also shoot bracketed, usually 5 frames, so I have at least 2 frames that are usually very long exposures (even on a tripod, mirror up etc.), such that DFS (Dark Frame Subtraction) kicks in. My problem is on the over exposed - they are very noisy. They use to be VERY clean. I made a post about this when I received the camera back. No one else has posted about any problem, let alone anything similar to mine.
Also, there are lots of folks shooting dusk and evening images that are bracketed, so there is more than a sufficient population that this would have affected if there were a fundamental problem. And I only have about 8,000 shots on the body.
Pentax warranty replaced the main circuit board and the sensor, updated the firmware, aligned etc. It appeared fixed until I went night shooting (for the first time after the repair) and the noise is back. Now - I am the ONLY one that I know of that has this problem, so I believe that its pretty much isolated to me (don't I feel special).
If there were similar problems with the K20 you would have had 20 posts about it by now. So, I consider myself the lucky winner of the random draw for a somewhat isolated/unique problem.
This board I think was the first with the mirror slap issue on the Kx and the green line issue on the early k7. Then the early SDM problems with the 16-50 lenses. So, if there was an issue to be found, it would have been posted here - already.
So, the moral of the story is to - Do NOT worry about it. You have a great camera with an excellent
model history. Go shoot pictures and enjoy yourself!!
PS - So am I looking to upgrade. No - I am looking forward to getting my K20 back so that I can go shoot the images again, this time after looking at my prior set and seeing some things that I did wrong.