I would absolutely love to replace my use of S3 with B2 as a backup for data stored elsewhere. Personally, I would much rather this storage to to a service that only does storage, rather than everything else that AWS does, so I don't have to worry about anything strange happening in a cloud service I don't use every day.

When they first launched B2, I inquired about ability to enter into a BAA (Business Associates Agreement) for HIPAA compliance and was told that it wasn't "on the roadmap". It sounds like B2 has come a long way on the compliance side. Would be great if they were open to this.