I'm ok[1] with them for "I accidentally deleted a file" type stuff, I would not suggest it for a full system backup. I'd suggest SuperDuper! for that if you're on a Mac; I don't have any useful suggestions for Windows or Linux users (sorry!). Of course, I use a full system disk image for "I need to be up and running at 95% capacity within an hour", and these offsite file services as, basically, long-distance undo, or for files I'd be unable to get back in some other form. Think tape storage: it doesn't need to available ultra-fast, but I need to have as many nines of certainty possible that there will be some way to get at those files again.

[1] though I'm switching off of them, not sure where to yet. There's some sort of issue in their client software that murders my home network, rendering the entire thing unusable while a backup is running. Their support was... less than helpful in trying to remedy the situation. For $5 a month and with the amount of stuff I was backing up, I can't blame them.