I've looked at B2 from time to time, but doing database blob storage over S3 or to disk and backing up database and files over rsync made us stick to our existing technology (eg Transip cloud storage which also charged 10 EUR/month per 2TB). One thing we didn't look forward to was having to reimplement cataloging and garbage collection for all of disk, S3 and B2, so we just stuck to a rsync hardlinking solution (which makes incremental backups painless)

having access to primary storage and cheap backup storage using the same S3 API will make us reconsider that and will probably make it worth the effort to dump our rsync-based solution for B2.