I use Dropbox. I initially tried to save to both my hard drive and Dropbox, but I kept forgetting to do one or the other and lost two manuscripts because of it, so now I just save to Dropbox. I tend to kill computers after a year or two (seriously, the laptop I'm using right now is my fourth computer in as many years, though two of the ones I've gone through in that time were hand-me-downs from hubby), so for me it makes more sense to just use some form of cloud storage.
All of my books that have been published, or at least submitted, are also in folders in my web-based email accounts, so if Dropbox fails, I can access those that way. (I just wouldn't have access to my works in progress.)