I did a clean install of OSX Yosemite on my late 2009 i7 27" iMac (reformat boot drive and install from USB memory stick, re-install apps, reinstate data - the whole 9 yards). I did this mainly because I had a few problems after 4.5 years and many Operating System Upgrades. My Mac was running a bit slow, it didn't like SD cards being inserted into the reader, so anyway I decided it was time for a complete referesh.
Oh how I regret that decision - in my experience Yosemite is the most unstable, bug ridden, quality control disaster in the history of my (8 year) involvement with Apple Macs. It randomly locks up, takes ages to copy file from one external drive to another (it just sits there with the beach-ball for ages), leaves icons randomly on the screen which sit above other windows and can only be removed by a reboot, wont write DVDs in the internal drive, etc etc etc
If my experience is anything to go by - think twice about downgrading to Yosemite until Apple have woken up and released a stable version.