What is it like to upgrade to OS X El Capitan?

If you’re a developer: much more disruptive than advertised. You have to undo the rootless mode if you do anything useful in Unix, which catches you unaware. Deprecating Java 1.6 (and I couldn’t get Java 1.6 working after a couple of hours) meant I switched to Java 1.8: much hilarity ensued, particularly with moving up to Eclipse Mars (4.5.0 is horrifically buggy; upgrade to 4.5.1 immediately). And the httpd.conf under Apache randomly got reset, so modules like jk_mod stopped loading for no discernable reason.