While I was hoping this was going to be "Yosemite"-"Snow leopard" it's not! I even did a clean install via usb stick. To many things still need work. Time Machine simply doesn't work right,doesn't play nice with MS Office (microsoft has said office 2011 users should stay with 10.10),Disk Utilities new interface is a major step backwards!,Side by side Yosemite default font is cleaner and easier on the eyes,Restart is slower,Just not as fast on older hardware (2010 11" air) I get the new beachball way more often,My fans ramp up slightly more often....need I go on and on.
While all the above might not be deal breakers,for me they were enough to go back to Yosemite.