Originally posted by Mareket we have equivalents of 300mm f/2.8, 450mm f/4, 70-200mm f/2.8
I agree the K-5 is a wonderful camera. But you have a false understanding of lens equivalence. Because many have, let me do one post to correct you on this:
The equivalent lenses to the DA 200/2.8, 300/4 and 50-135/2.8 (the lenses you obviously have in mind) read:
FA 300/4, 500/5.6 and 70-200/4 (rounded).
Such lenses exist (in general) and are no bulkier or more expensive than the DAs. In fact, they have an identical diameter in glass, and they do so for a physical reason.
The advantage of FF is that you can find lenses where equivalent lenses don't exist for APS-C.
One reason why people are confused with lens equivalence is that they forget to scale ISO to scale with the square of crop too. Not doing so would create images with different exposure time, different amounts of noise, different DoF or different diffraction blur, i.e., images which aren't equivalent. The F-Stop, ISO and focal length must change all three when switching the body between APS-C and FF, in order to create an identical image.
So, while I agree the K-5 is fabulous, the reasons why it is are elsewhere.