Downloaded and installed this last night. Tried booting it this evening and the bar gets about halfway along and it then justs stops - I got bored after 30 minutes of waiting. It won't even boot into recovery mode or from the Yosemite installation USB so I suspect it's something fairly fundamental that's failed - I guess I can't complain after 6 years and a heck of a lot of use over those years. I'm just so glad I took a backup before doing the install, I've got thousands of photos that I would hate to lose.
Thing wouldn't even boot up properly in Safe mode. After over 4 hours of trying various boot methods I've finally got into Recovery and it's currently restoring from my last, pre-El Capitan, back-up. Should all be done in 4 hours...... I've got everything crossed.