I had way too many beachballs, little pauses and hiccups. I use my computer to actually do stuff instead of staring at smoothness of scrolling in Safari. I had Adobe Qtkit hanging, FCPX beach balling and crashing, audio apps freezing... After Effects CC doesn't run on 10.9 at all. I had none of these issues on 10.8.5. I know this may be just related to an insulated issue on my machine, but since upgrading is the official supported way to install Mavs, it didn't work out for me. And I tend to run pretty clean machine. Please don't tell me to repair permissions and stuff, I actually write printed books about ios and osx for a major publisher and know my way around Apple stuff. This is just a little reminder for anyone who would like to upgrade on production machine. Don't yet.