I've always done clean installs. I've just assumed it would make things run the way they're intended and that an upgrade must somehow mix the old and new (that may not be true at alljust a "feeling"). But I just got my computer back from a repair a month ago and set everything up again after a clean install, and it honestly takes a few days to get things set up the way I like them. My computer doesn't even start to feel like "home" again for a few weeks. There are so many things to do (settings to change, programs to download, install keys to find, not to mention I have to call Microsoft tech support each time for them to issue me a new key for Office because after all my reinstalls it looks like I've tried to install it on too many computers). I've had quite a repair history, and after each repair I've done this. I'm considering just doing an upgrade of Mavericks. MobileMe used to sync system preferences, which made clean installs less of a chore. I guess the other reason I'm considering it is that nothing is not working right now. In the past I've done clean installs after something had gone wrong. I guess I'm curious to know what other people are doing.