I use Dropbox for my main body of work, and access everything from there. This allows me to read my work on either of my computers, my iPad or my Blackberry.
To some degree it is a backup too, since my computers keep a local copy. So if Dropbox went to the digital bit bucket in the sky, at least I have a local copy. However, if Dropbox just deleted my files and then that delete synchronized and deleted my local files, I would be pooched. If I did not have another backup mechanism, which I do.
So from my Mac, the Dropbox folder is then backed up via Time Machine.
Finally, to protect myself from my own incompetence, I keep versioned files. So I have a history of files Story1.doc, Story2.doc, Story3.doc. If I butcher something inside my document and then save that, then that synchronizes to my other devices, and then my Time Machine backup failed, then I still have a version from last week or whenever.
Off site storage - check - Dropbox.
Aged versioned files - check - Story1, Story2.
Data in multiple places - check - Dropbox, PC, Mac.
Ability to step back in time and restore - check - Time Machine.
Me, I distrust on-line backups, and don't use such services. In addition to writing, I am dependent on computer files for my business. I keep things on several different flash drives, and three separate computers. I have had problems with the reliability of external hard drives, and no longer use them.