I have the K-01, the mirrorless camera that did not succeed and I doubt that I will ever buy a non-mirrorless again. My K-01 has good battery life, top-notch focus accuracy (autofocus + MF with focus peaking) and it's silent. I also bought an eBay loupe that you can stick to the LCD screen (via a magnet). It adds to the size but it's so cheap that you can always carry it with you and not worry. It's a big help on sunny days and you just take it off and put it on again with two fingers.
My K-01 is bulky for a mirrorless (but I don't care), its focus speed is comparable to my Ds (not good), the interface is sometimes inconvenient (e.g. the histograms, the directions on the wheel).
But if Pentax would update it with modern technology ... an electronic viewfinder, although that would add considerably to the cost of course ..., a higher resolution LCD screen (focus peaking etc.), better AF speed, ergonomics in line with other Pentax camera's, then I'd buy that update, even if it's bulky (preferably because of the battery).
But for me the most important advantage of mirrorless is focusing accuracy. I can take f/1.4 pictures with my FA 50mm and not worry too much (although it's still a hit-and-miss because of this moving world). And I can buy a Sigma lens and not worry either.
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A.