I took a lot of wonderful shots with my old K20D, and it was so dependable and trustworthy too. I can't recall that it felt any more solid than my K5 or K5IIs, but it did have a nice feel to it also. I used it in some studio shooting for a while and it performed very well. I do think the WB and color is better on the K5 series, but that's just my thought.
K20D
I'd be hard pressed to say anything negative of my old K20D, it was a winner!
The low light edge most certainly goes to the K5 series. Depending on your needs that can make a huge difference.
I think theK10D rendered the most appealing photos to my eyes, but you sure had to watch the ISO and keep it low to get what you desired. If you could stay below 800, it was magnificent!
K10D
Regards!