They're as good as you keep your password safe. I don't know if they encrypt your data but I would imagine they do not if your password is recoverable.

I don't use the free ones much, Gdrive and Skydrive only but even then it's mostly when I want to share a file with someone.

Everything I backup is on a paid service, encrypted with a long key only I know so even if someone ran off they'd have a bunch of scrambled stuff and nothing of value

By the way, SkyDrive from Microsoft doesn't allow anything porn or nude uploaded.