I had a K10D and upgrade to a K20D. At first, I didn't think it was worth it. It just seemed a little faster (it seems to have better
AF with my DA* 50-135) and a little more accurate, the screen is noticably bigger, AWB is a little better and I was looking forward to using ISO 1600 some day.
Well, when crawling on the ground taking pictures of mushrooms last fall, I used live view for focusing. It was perfect for that and I got great pictures (the composition I wanted and perfectly in focus). I coudn't have done the same just using the viewfinder.
When photographing something unfolding over time (traffic, a party of kids, etc), stick a wide angle lens on the camera and use interval shooting. Stitch the pics into a video and you'll be amazed.
For me, ISO1600 on the K20D is way better than my ISO800 shots on the K10D! I took a ton of shots at a concert last week, all ISO1600 and the quality is fantastic.
I love the extra battery life when I was shooting the concert (has passes for a meet and greet before the show so I was actually at the event taking pictures for 8 hours on one battery!).
So, the more I use the K20D, the more I like it. I found it well worth the upgrade from the K10D.