I believe the K3 is also the first to use colour based AF. I broke my K5 in easter, got a K3 from my insurance company, broke that this October and got a second hand K5II replacement - long sad story, but the point is I have tried all three!
I am going to get another K3 when I can afford it. In short, it is the only camera I have ever had that was able to focus correctly, and hold the focus, on birds in flight flying past vegetation. I noticed the difference very acutely because I broke + replaced my K3 in Florida while shooting a lot of birds. The downgrade was noticable. After I came home, I had a photoshoot for a hunting article, and I returned with a whole series of teals flying past autumn coloured trees - could have been pretty, but not a single one nailed the focus.
I am not saying the gear was to blame - I am sure the photographer was. But in my case at least, the K3 was a leap forward in terms of AF. I miss it...