S3 (Amazon's Simple Storage Service) itself is more of a service backend for SaS and 3rd party developers. As a developer who's worked on S3 before, it isn't the ideal platform for an end-user back solution. If you are a developer S3 is great, considering the substantial cost savings you get over services that are built ontop of S3 (like Mozy).
Plus, you can ship them harddrives and they will upload it to S3, rather than spending time and bandwidth uploading the files directly--though almost all online backups don't charge for bandwidth, just storage.
There are 3rd party tools and utilities that make backing up to S3 more manageable, but if you're looking for a robust solution rather than a "roll your own" then I recommend going with a service that is built ontop of S3, rather than going to S3 directly.