I am putting in an order on the K-1 because I have been telling Pentax that if they ever made one, i'd buy it. Other reasons aside, one of the better ones for me is, I want to support their decision to make a full-frame and hope they continue development on full-frame products and Lenses. I would LOVE to see a weather resist. pancake for the K-1. It would make such a great portable full frame!
I want the low light performance as well, and am going stick a few M42 screw mounts on it also. Because nothing says crazytown like a 1100mm USSR mirrored lens found on the cheap!
I know this is said, but the best lens is the one you have with you. I had a hard decision when looking at the K-1. There are some very nice Sony cameras that will fit in my pocket. The are upwards of 800$, but I would take it with me everywhere. My compromises are, I am going to force myself to not let a couple pounds deter me from taking it on a hike or weekend backpacking and that will mean I need light lenses. I have some glass that I love. All of my nice glass, is heavy glass. I have taken some of my best photos with the DA 40mm f/2.8 XS pancake. I don't bang my camera on things and it seems lighter, with the XS equipped. For me, I decided I either use my camera and lenses, or sell them and get something I will use.
Not perfect, but I need fewer hobbies.
I have also noticed, nice primes are lighter than the DA* zooms, so there's that.
Sometimes I also play a game of "What neat things can I shoot non-traditionally with these one or two lenses" Which is great, until I reaaaaaly want a telephoto.
I am also looking forward to having a body that will outshoot my lens. I've never done it before, and am curious.
I also want to 3D print a pinhole for it and another that you can slide cheap gels into (the theater light kind).
Cheers,