I had my first weekend photo trip with the K-1 this past weekend. I was particularly interested to see how well I handled the extra weight of the camera + unmodified 60-250, and of course I wanted to see how the images turned out.
Most importantly, I love how the images turned out. The K-1 stabilization seems to have made it possible for me to get sharp images at slightly lower shutter speeds than I'm used to, even while hand-holding a much larger and heavier lens than the 55-300 (my usual lens of choice for these excursions).
I also loved having more pixels to work with - because I often crop down with these images at the sanctuary (because I can't zoom or foot zoom to get the composition I want).
The vignetting that comes with using the unmodified 60-250 on some settings wasn't nearly as limiting as I expected it to be - I was conscious of trying to shoot a bit loose (something I do anyway), so that vignetting that I couldn't fix could be cropped out, but when I reviewed images I often found that the vignetting only affected the very outer corners of the shot, so there was little adjustment/cropping needed. And the 60-250 works well most of the time for the kind of images I'm making.
Images at high ISOs were significantly less noisy than what I'd experienced with the K-3. This is a HUGE plus for me, because I often photograph without flash in poor light (at the Wild Bird Fund hospital, for example).
The negatives:
Well, that lens/camera combo is significantly heavier, and for someone like me with back, shoulder and hand difficulties, I noticed it. I used the Cotton Carrier vest to relieve some of the weight when I was just walking, not shooting, and that helped. But I think I need some strength training - and I'm not sure I'll be able to handle a heavier lens.
I did miss the extra reach that a longer lens would have a few times.
But all in all, that's a very short list of cons, compared to the pluses.