I’ve been running OS X 10.9 Mavericks for a few days now. Overall, I’m impressed by the speed improvements and how trouble-free the upgrade was. There aren’t any major new features (other than a few programs added from iOS), but it’s a solid update and probably representative of the yearly update cycle Apple is planning to use from now on.
(I don’t understand why so many people insist that 10.9 isn’t a new version but an upgrade. Although it’s true that many people like me will install it as an upgrade, that is a meaningless distinction. Almost every operating system has offered upgrades for existing users. By that reasoning, only Mac OS X 10.0 back in 2001 was a true version and everything since has been an upgrade.)
The main Mavericks compatibility problem I have seen mentioned is with Apple Mail. Mail has always had problems working with the nonstandard IMAP support used by Gmail. In its latest version, Mail has special support for Gmail, treating it differently from other IMAP providers. Unfortunately, this improved support has had the unintended side effect of causing problems for those people who had already customized their Gmail setup to work better with Mail.
In particular, it was widely recommended that you hide the Gmail “All Mail” label to prevent Mail from downloading two copies of each email. But this trick doesn’t work with the current version of Mail, and can lead to email vanishing after you move it (change the label, in Gmail terms).
The fix is simple: re-enable “Show in IMAP” for the “All Mail” label in Gmail. If you (like me) never hid the “All Mail” label, then you probably haven’t experienced any problems and don’t need to do anything.
Some people are reporting other minor problems caused by Mail’s improved Gmail support. For example, if you like to use the Delete key to archive a piece of email, then you will likely be disappointed by the new behavior. (Personally, I’ve never understood using the Delete key to do anything other than delete.)
You can see a lot more information about Apple Mail and Gmail compatibility problems at TidBITS.