I use Carbonite online backup, and although I've not had a major crisis, when I have needed to restore a few files it has saved the day. Also it's cheap.
It does have disadvantages.
i) For quite a while my laptop was running slow and crashing, and miraculously, when I uninstalled Carbonite - problem solved. Since the re-install, it has been running well.
2) It does not back-up external drives. I keep all my recent files on the internal HD, only moving files over to the external drive when they are also safely backed up elsewhere.
All in all, I'm reasonably happy with it.