Personally I don't bother backing up my OS in most cases, as I know I can reinstall the OS and most of the apps I use daily within a few hours.
What I absolutely DO backup is documents/files that can't be replaced if lost.
Tend to use Crashplan (To a machine I own) and SpiderOAK (Cloud) for backup.
Testing cloud drive on one of my machines to backup a few virtual machines to OneDrive and Hoping they'll be able to support Hubic at some time, this tends to be stuff that I'd like a backup copy of but it wouldn't be the end of the world if I lost it also the files tend to be rather large so I wouldn't want to use Google/Amazon.etc as the costs would quickly mount up.
Actually also looking for an alternative to Crashplan for backing up to my own server, but the problem is most of the Apps don't do encryption + incremental uploads/Block level. The ones that do tend to be enterprise focused and cost a fortune.