Apparently they run a limited gamut of focusing screens. Sourced either from Canon or Nikon. For my Df, I had the option of several Canon screens, including the "S" all matte kind, and a few split image/microprism versions (from F6, FM-3a).
The "S" Canon screen is uniformly described as very dark, but few if any reviews say it is coarse-grained which really is the most important aspect. Thus my choice of the K-3 was easy.
After having used the K-3 in my Df for some days, I'm in general happy with it. Matrix metering works well enough, centre-weighted is generally good although a small global adjustment improves the situation, and spot metering is more erratic no doubt due to interference with the split image and whether microprisms blacken out. For the really fast (up to f/2) lenses, spot metering can be trusted, thereafter, it's more a hit or miss affair.