I use DropBox for sharing of non-sensitive files. I like it. But the following article (and comments) would worry me if i were using it for backups of sensitive information http://tirania.org/blog/archive/2011/Apr-19.html

I have been really happy using JungleDisk with RackSpace storage for my backups for a year or so now. Easy tools (I'm on a Mac), scheduling, encryption and data compression to minimise cost and bandwidth use. Easy control over the number of revisions of files to retain.

Personally I only keep minimal revisions in the cloud - it's only purpose in my plan being for disaster recovery or occasional remote access. I keep multiple revisions locally through time-machine. I totally lost patience with the UI on EMC's Retrospect.