I just upgraded to the K-1 from the K-5. I always wanted a K-1 but worried about the extra size and weight plus the expense.
Wow, it blew me away. So much of an upgrade in every area, by leaps and bounds. Being able to use the FA Limiteds as intended with full fov and the big viewfinder is amazing. Its like everything is new again... The 77 which I didn't enjoy using too much on APS-C with the long fov is a revelation on FF. An adjustment using my APS-C favorite the 31 but now I can see what it was really designed for and the K-1 body must have been styled to pair with the 31, together the exterior look is perfect...those two lenses are just really something extra special on FF.
Within a couple of minutes I could barely tell the difference with the additional size (which is not that much), the handling and controls were so familiar. It does weigh a bit more but it is not a big deal at all.
The image quality is amazing, what a camera... If you get the K-1 the moment you mount a favorite lens and take a shot you will be in love and so happy with your decision
For a DA15 replacement (a lens I used to love), you can consider the M20 F4. Haven't tried it yet but it seems to be similar in many ways and to have a great character and size.
I was on the fence before but there is no way I would go back now!