Spider Oaks good qualities as an offsite backup service are too numerous to list (please do have a look around their website to catch anything I might have missed):
The security aspects of this service are what really sets them apart. They have set it up so no one at their company has any way of accessing user files, which are stored encrypted on their servers. You can read the technical details on their site, but the only one on the whole planet who has all the necessary information to decrypt your files, is you. A corollary of that is that if you lose your password, there is no recourse. You lose your files on their server. They cannot "reset" your password.
There is only one hole in the design of their security, and that is that they have not Open Sourced the backup client you run on your own computer, so we must trust them when they say that our passwords are never sent back to the server, and that there are no other back doors. (Just like any other closed commercial application, for that matter.... But until an Open Source competitor appears, Spider Oak effectively has no competition.) They have been around for a while, and they have some quite significant endorsements on their website, so I am inclined to believe them and entrust them with my own personal files. This is the first service I have found, ever, that I can say that for.
I have been running their Linux client for several days now, and it is both highly polished and very stable. And, for server administrators and cron users, the client can be run headless from the command line.