ZumoDrive is an awesome idea -- be truly extra space, not just a mirror of a folder that has to exist locally, too. But it's been extremely unstable on Mac OS X for me. I wish it worked and toyed with it for a long time, but it kept doing things like hanging my entire machine at startup for 5 or 6 minutes until it crashed and quit and the rest of the startup sequence resumed. Or showing as mounted but acting as if all my files are gone (they'd always still show up in the online interface though). Great idea, questionable execution so far. I'm still open to hoping it gets stable for mac and coming back to it though... but not now, not as a primary location for important information.
It's the same story on Windows. I'm closing my account over the next few weeks after having used it for a year. It's simply not stable enough, and I've lost patience.