I was considering to mirror my cold storage backup (12 bay NAS with a few hours uptime per week) to tape for long term archival of private data (e.g. photos) or stuff on the internet I think is worth having a backup of (e.g. the http://unrealarchive.org).

But the tape storage requirements and danger of running into trouble with the reader (especially considering I could get a misaligned one when buying used, and if that one breaks => tough luck) convinced me this isn't a good choice. [edit] For the same money I can just get a bunch of additional disks, and use two for simple, redundant, offsite backups; and the remainders to swap out old disks in the NAS pre-fail.