I upgraded from K-5 to K-5IIs earlier this year, my experience is that improvement in AF has make me enjoy using my fast lenses much more as it is more accurate and focus much much better in low light with all lenses, in fact that's the major factor that trigger the upgrade. The other thing about the IIs is the lack of AA filter makes even shots by 18-250 superzoom look sharper, I shot a friend's wedding with both K-5 and K-5IIs and the difference in AF in low light is night and day and the sharpness difference is noticeable(I used DA*16-50 and Tamron 70-200 f2.8 between them, switch around when needed), and convinced me to sold off the K-5. Overall IIs is a positive experience for me.
I have tried K-3 in store, overall performance/responsiveness is improved, AF with screw drive lens is faster than IIs(much faster than the K-5 classic) too, but a few things prevent me from getting the K-3,
1) I will have to get a new battery grip, I already have the D-BG4 for K-5.
2) I will have to upgrade my PC and Lightroom( I still uses LR4.4) /get more HDD to deal with the newer / larger files(I shoot RAW+)
3) I don't print large, so the increase in resolution is lost to me.
4) I will have to develop a different set of LR preset for K-3 RAW, the ones that I developed for K-5 RAW only needs minor tweak to work with K-5IIs files, mainly to tone down on sharpening
5) Cost, the money I saved by going with a lightly used K-5IIs instead of a new K-3 enable me to afford a Canon 5D with 50 1.8 to tame my FF itch.
Truth to be told, my skill as a photographer has not outgrew the original K-5, but the irritation of AF has spoilt quite a few occasions for me so for a very minor cash outlet (I sold my 3.5yo, 63k shutter K-5 not long after I got the IIs to offset the cost)I am able to enjoy taking photo for a long time.
Actually looking back at my reasoning it's a lot to do with me dealing with a very tight budget, if budget is not a concern for you just get the K-3 and enjoy the best DSLR Pentax has ever made. My 2cents