Ranked in these QuestionsQuestion Ranking
Pro Can take super low noise, high sharpness images with Pixel Shift Resolution enabled
K1 has an alternate shooting mode that can significantly reduce noise and increase sharpness. It's called Pixel Shift Resolution. Once enabled it will take 4 shots moving the sensor by 1 pixel each time in order to capture all color information for each pixel instead of distributing it over 4. A visualization can be found here.
Pixel Shift Resolution shooting mode takes longer as it's taking 4 shots instead of one and requires a couple of seconds after the shots have been taken to process the images into one so it won't work for sports or action, but it's great for landscape photography.
Pro In-body image stabilization allows stabilizing fast prime lenses
You can potentially do night photography without a tripod by using a fast prime lens wide open with in-body image stabilization.
Pro Great interface
Pro Good dynamic range
Pro LCD can be pulled out of the cradle
K-1 has a cross-tilt LCD.
Pro The K-1 + one of the Pentax Limited full-frame, all mechanical lenses is the dominant design in photography
It's not the camera alone; it's the camera and the lens together. Pentax's best lenses are the FA31/FA43/FA77 Limited series. There is no other camera + lens combination that can compete with the K-1 + one of the FA Limited lenses on a price/performance basis. It is not even a close competition.
Pro Built-in GPS
Great searching through picture libraries by location.
Pro Manual sensor shift allows taking tilt-shift photos with any lens
Pro Has LEDs that illuminate key parts of the camera
There's an LED that shines light on rear controls, there's another one that illuminates the lens mount, and an LED that highlights the card bay and remote release port. These can be really helpful in the dark.
Pro Automatically fixes horizontal level
Con Autofocus could be better
Autofocus won't be able to consistently keep up with fast-moving subjects so it's not the best camera for action or sports. Additionally, it only has 33 AF points which by today's standards is on the lower side of things and they're not spread around as much as in other cameras.
Con Video capabilities could be better
It's reasonable to expect more than 1080p30 in a 2016 flagship camera.
Con Limited lineup of full-frame lenses
While Pentax has among the best selection of lenses created for APS-C sensors, this is their first full-frame camera and there aren't that many of them.