I have been using Box.net for a couple of years, and one of the main features that it lacks is the ability to sync uploaded files automatically to all of my desktops/laptops. It has finally come http://www.box.net/services/box_sync.
However, it currently doesn’t support Mac, you can add yourself to the notification list when the support for Mac is ready here http://sync.box.net/boxsync/mac.php.
Looking forward to the Mac version.