I moved to Pentax from Canon. They are coming out with a new FF this year using the A7rII sensor which also may have their Pixel Shift technology which would put it on par with detail resolution of a Medium Format.
I love Pentax. I am only using one of their lowest end models, the K-50 but it has more features than my Canon 60D did (minus flip out screen). I think it's a better camera and the body only cost $300 new. You get a LOT of bang for your buck with Pentax.
They have features others don't, like IBIS and with the GPS unit it will communicate with the camera and allow the sensor to actually track the stars up to 5 minutes with UWA. I think that's pretty damn amazing and the new K3 II has the GPS unit built into it.
Their weather sealing is top of the line too. I have seen somebody take a K-50, their lowest end model and put it under water in a pool and take pictures with it! That's just insane. I wouldn't try it, but I don't think any other SLR could handle it that well, especially in this price range.
I also like the DR with the Sony sensor, I am able to pull more detail out of shadows than I could with my 60D.
I also like that I am not supporting one of the larger names. So, there are some downsides. One, selling your gear if you no longer need it, the markets not nearly as big so selling stuff is not as easy. Buying is though, you'll have more bargaining power. Third party support is not all that great though Tamron is said to be in line to support the new FF.
Lots of old glass you can buy that's really nice and cheap and on the digital body will allow you to have the stabilized image. I can't think of too many downsides. I say take the plunge!
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