Always remember that sync solutions are not complete backup solutions.
They protect against a local machine failure or theft of a laptop for example but if something wipes the contents of a file then the wiping of the file will be copied from the machine to the cloud sync storage.
Of course the other problem is that a unsecured laptop using a sync solution is vulnerable to a thief deleting your files before you realise that your laptop has been stolen.
Really important files should be properly backed up and services like Cryptoheaven provide a good secure solution for this http://www.cryptoheaven.com only problem with them is that if you lose your password even they can't get your data back
