I often find the 50mm a touch too short for me and after borrowing a 77 ltd for an hour I had to have one. Build quality is outstanding in every way, it is light and portable as well. An 85/1.8 is considerably larger and heavier. Image quality is as you would expect, sharp enough at f/1.8 and very sharp by f/2.8. At f/4 and 5.6 resolution is outstanding. More importantly, contrast and colour are fantasic, rich, contrasty results with very little need for processing.
The bad news? It's expensive, AF is slow and noisy (however the long throw MF ring is a joy to use) and there is some purple fringing/CA problems at the widest apertures. OOF areas can suffer from severe cyan fringing.