Currently Im finding Sierra to be the best yet since Yosemite, when I bought my iMac.
Ive only had apple mail issues, but I think Ive solved that problem.
It seems so much more stable, I even had time machine do a backup to my external hdd in sleep mode, thats a first!
Not had any beachballs yet like I was getting on el cap (touch wood).
And Ive gained nearly 10gb of space!, I had about 463gb free on el capitan, with Sierra I have 472gb. not quite sure where I gained the extra.
If anything I have noticed it takes longer to wake from sleep? it was more or less instant on el cap, now when I click the trackpad to wake it, it seems to take a good 4-5 secs.