I spent a day upgrading my MacBook Pro from Mavericks, OS X 10.9, to El Capitan, OS X 10.11, and then downgrading it back to Mavericks. I needed to downgrade my OS back to Mavericks as the system fonts on El Capitan produce horrible glyphs on my older external monitor[*] and, further, my 52 years old eyes were not able to adjust to the high contract between the visual strength of the background and the visual weakness of the glyphs. No amount of hardware adjustment or software calibration resolved the problem. Apple's visual design drive to near invisible text -- on OS X and iOS -- is going to push me away from their products.
[*] Acer B243H 24" Widescreen LCD Monitor