A little late to the party and I don't have a Df, but I do have a 105 mm f/2.5 pre-AI lens that I have been found very enjoyable.  
I purchased it for a fair price, but it wouldn't focus properly, one of the helicoids was jammed.  Thanks to Richard Haw's website, I was able to disassemble and get it back together again and working quite nicely.  I AI modded it for use with a D50 and D70 that I had at the time and added a Dandelion chip.  Since then I acquired a D610.
I have two pre-AI lenses I modified, but this is the only one with the metal scalloped focus ring. It's a shame they didn't keep the metal focus rings and moved to the plastic/rubber rings.  
This is the 6 aperture blade variant with a min aperture of f/22.