I love dropbox - not a backup in the traditional sense but a fantastic tool (and I think you can purchase a pack-rat option that it is more like a backup - otherwise, if you delete a file it will do so on the server and all other machines (some limited restore is available). I use it mostly to synchronies between all my computers.
What I use for true online backup is CrashPlan (http://b4.crashplan.com/) runs very smooth in the background and never had a problem (I tried recovery just to try it out and everything was smooth). I am using them now > 1 year and I'm still happy.