Mac users, stick with Mavericks. OS X Mavericks isn't great, but it is usable unlike every other modern operating system. It retains some of the good Apple design sense that originated with Steve Jobs, particularly skeuomorphism. Skeuomorphism makes the user interface intuitive and attractive, so naturally the modernist designer scum hate it, and this includes the chief designer at Apple today, Jonathan Ive. So Apple's latest operating system, Yosemite, removes all skeuomorphism and the user interface is horrible. And following modern trends, this operating system is unstable and crashes frequently. And Safari and other apps have been changed to be considerably less usable.
Of course as a general rule, one should try to avoid upgrading anything since virtually everything in the modern world is only getting worse. But this is especially true for Mac users. If you have a Mac then NEVER UPGRADE from Mavericks to Yosemite.