I'm too far gone to go back to Yosemite, but El Capitan is Vista level disaster on MBP and MP of mine. USB3 ports are no longer full speed - my Blackmagic Ultrastudio is completely unusable at the moment, timemachine backups fail 12 times a day, removal of HDD formatting for third party file systems (NTFS etc) from Disk Utility was as close to professional suicide as I ever got, the bloody printer/scanner utility works only once then never wakes up network scanners any more, prints eventually at some point work, but the printers have to be rebooted for the scanning to work again, auto updates attempt to reboot me every day, despite disabling the feature, safari passwords have amnesia and don't queue up half the times, you check preferences, they're there, you go to the page - it's not suggested, when you enter it manually and Safari asks again if you want it to remember it (and the stupid bug where it asks you to remember the password and freezes progress instead of logging you into the site first is still there) and to be honest I don't even know what I gained by upgrading.