I shoot PEF.
I recently, due to some major hardware and software changing, considered switching to DNG. After recieving some good information regarding both formats I have decided to stay with PEF.
Some key points as to why..
Can fit more images on card.
Can write more images to a card per minute, though DNG does write more initially until buffer is full. Two images more, that's it!
Can always be converted to DNG at a later date.
Adobe products work identically with PEF as they do DNG.
Considering future Pentax cameras and what software they might ship with, PEF will be supported guarranteed.
Many claims as to how DNG somehow produces better/sharper images - are all false.
No import time differences using Adobe Aperture - both are equally slow.
And finally, not really a reason why but, Godfrey DiGiorgi shoots PEF as well..