I used to use Jungle Disk until I found out they still bill you for bandwidth costs even if you use your own S3 bucket so you effectively having to pay double for bandwidth costs.
any hacker worth their salt can just use rsync or something else. and you can somewhat trust your remote host with openSSH.
if you are already going to pay for storage/bandwidth? why settle for a dumb client and doubtful privacy?
dropbox i can understand on the price issue to be more appealing, but if you plan to backup private documents, this is not a good selling point. more so in recent fear mongering campaign against them.
I'm paying the lowest linode with backup and just have a cron job on all my devices (home/work PCs, android, openwrt router) sending rsync diffs there. and as an added bonus i also use that host to host my private git repositories and run mysql slave for an offsite backup for my personal projects data.
...not extremely cheap, but not extremely insecure.