Yes, I've regretted upgrading to Yosemite when 10.10.1 came out... First thing, Filevault managed to crash and corrupt my hard disk. Had to restore from my backups (not a huge issue but a waste of time).

Then, I cannot connect to either iphone or my wife iphone's hotspot without turning the wifi off first and then turning it on again. I have the same issue on both a macbook pro and a macbook (but didn't have that issue before Yosemite).

And in term of performance, my macbook pro is nowhere as performant even with the latest Yosemite update as it was before. I also have noticed a strange issue. On yosemite, whenever the battery reaches 5%, everything slows to a crawl...

I feel that the last really stable OS from Apple was Snow Leopard