Bike Bell -- NEX-5N & FD 85mm f1.8. Click for larger.
For some reason, 85mm lenses tend to be very expensive. I think partly because most of them are quite fast with maximum aperture of f1.2 to f2, with a few at f2.8, or even f3.5. On full frame, 85mm is a portrait focal length, hence most of the 85mm lenses have large maximum apertures to soften the background to make the subject stand out.
Autumn Leaf -- NEX-5N & FD 85mm f1.8. Click for larger.
From experience, most of the 85mm lenses are very good, especially the Takumar SMC 85mm f1.8, and the Leica-R 90mm f2 Summicron. The Canon FD 85mm f1.8 is also an optically good lens. It's already quite sharp wide open. By f2.8, the lens is extremely sharp. The Super Spectra Coating (SSC) does a great job of reducing flare as well as improving contrast. The bokeh is quite nice with 8 aperture blades. I would rate this lens in par with the Minolta MC 85mm f1.7.
Fall Colors -- NEX-5N & FD 85mm f1.8.
One of the best things about the FD 85mm f1.8 is that it's cheaper than most other mounts. The Minolta MC 85mm f1.7 costs more, and the Takumar SMC 85mm f1.8 is now priced up the wazoo. It's still good value for the money. The only disadvantage is that it, and the Minolta MC version, can not be used on Canon bodies without adapters with correction glass, which also mean they can not be used on full frame. But, they work wonderfully on M4/3 and NEX cameras.