I was periodically backing up to an external hard drive that I then stored in a fireproof, waterproof safe (in the event of a catastrophe). The problems were: I had to remember to do this, it was (obviously) a manual process that was fairly time consuming (I back up all music, video, and photos), and files created/modified between backups were at risk in the event of loss of some sort.
I have since installed a backup drive (Drobo) that stays put at the desk and subscribed to CrashPlan for online backup. It runs $50/year for unlimited storage space for 1 computer, including any attached drive. Like the poster above, you can set it to perform backup during non-peak hours, so it is pretty foolproof once you get it set up. As with Carbonite, you can choose specific drives/folders/files to backup.