My mum uses it. It's pretty mainstream.

I've got both my parents using it, mainly just as a file sharing service (drag file into dropbox, get public URL, paste in email). It's easier for them to understand than a file sharing site like Rapidshare.

I've gotten some of the least techie people I know using Dropbox. It's surprisingly one of the few new tech products that's really just caught on with computer users of almost every skill level and sophistication.