I am a very happy K-1 user but there is one thing that frustrates me. The 15-30mm f/2.8 lens is a great lens but it does't fit my purpose unfortunately. I am a landscape photographer and travel a lot and this lens is too heavy for my use and the main issue is that it doesn't have any filter thread.
Canon makes a fantastic 16-35 f/2.8 (iii) and a 16-35 f/4 with weather sealed features and so, Nikon as well, and then Sony/Zeiss. They all take filters (most have 82mm threads that work well with the 100mm Lee system).
I don't mind it it is a third party lens (Tamron, Sigma) but this is something that I really miss at the moment. I do have the Tamron 17-35 f/2.8-4 that almost does the job, but it is not weather sealed, it's design and construction is 10y old and it is not the sharpest at wide aperture.
So please, Pentax, listen to frustrated landscape photographer and do something about it...
That was it... thanks chaps