I've had weird issues with my whole Dropbox getting deleted - in the middle of the night, with my machine off, have never figured out why - and while your Dropbox can be restored through the web interface, it's just one file at a time. In the case of a "mass delete" you actually need to e-mail support and generally wait a couple of days before they restore everything, which is less than ideal.

The last time it happened (the third time!) I wrote some scripts myself using their API to do the restore rather than wait on support. It really doesn't make for an ideal backup solution, though!