This lens suffers from the deserved reputation of the 105/2.5. In fact, wide open, it is not so good, but usable under the right circumstances: avoiding high contrast edges, processing in B&W... f/1.8 is marginally useful for isolating distant subjects.
Stopped down, sharpness is excellent, at least as good as the 105/2.5, and the field is flatter. Also, as mentioned in other threads, the 105/1.8 does not flare a lot and, especially, withstands light sources in the image field, making it an excellent lens for stage shooting.
Today I went on a stroll with that lens only. My copy is optically ok, but the helicoid is not sufficiently lubricated, leaving me with some dry friction and making the manual focus process disagreeable. That's one reason why I do not use it as often as I should.
All shot with my 5-years old Df. May it live forever.
First three, f/4, f/4, f/1.8