re: Michael C
Everything you say about the evolution of tape is true. But it also can be said of spinning disks as well. The data is tightly packed there as well. LTO has been very reliable with me since Gen1. And call me heretic but QIC was very reliable as well, last year we still had a few 125/525MB drives working daily backups on tapes that had had numerous reinsertions and rewrites in dirty environments (back rooms in fast food restaurants...)
RDX is basically good idea and simple to use, and if you only need a couple of cartridges, it's fine. But if you need the speed or capacity, the RDX is no go. For the high price of a cartridge you could actually pop a same size hot swap SATA hdd into the server and backup to it. (I wouldn't recommend this though!)
On the low end systems tape still has its place. The Unitrends D2D systems you are recommending here (for the third time now!) are in a different league and I agree that if you need to move 10 or more TB's replication or some other technologies could suit better.