Pretty disappointed with Yosemite on my 2.66GHz dual core i7 with hyper threading, 8GB 1066MHz RAM, 7,200RPM hard drive and 512MB dGPU set to always on. UI stutter where there wasn't before, even dashboard... which should be the same since functionally it isn't any different, but it lags now. Mail keeps asking for my Yahoo mail password and I keep inputting it in correctly over and over and it refuses to accept it and isn't working after re-adding it either. iTunes songs and music files skip probably 20+ times per song when I play them like an old CD player. It didn't used to be an issue with Mavericks. Also when I type in search bars on the Finder it can lag really really bad, like a several second delay before the characters show up. Also RAM usage is way up around 4GB on idle. Not an issue since I have 8. Opening apps like System Preferences is wayyy slower than before. The same goes for iTunes. Google Earth runs okay, surprisingly. Before anybody says "it is so old it isn't designed to run well." My machine gets about the same Geek Bench single core as a 2.66GHz Mac Pro. It ought to run okay... my theory is that Yosemite is to blame. This post isn't about the UI... I'm pretty impartial to it though leaning towards dislike. The "World's Best Operating System" ought to have optimizations up the wazoo like we've seen in past versions... but the results in, and this OS makes your Mac slower, period.