More about backup: Drobo and Time Machine

A our backup disk slowly ran out of space I finally followed the advice of the TWIP podcast and bought a Drobo (which is a sponsor of the TWIP podcast). But it's really a pretty cool device and in my opinion tailor-made for Time Machine. Here's why: the Drobo is a disk array that allows putting up to four disks together into a redundant "super-disk". This means that if any one disk fails, it can be replaced - online - without loss of data. As opposed to traditional RAID arrays, the Drobo allows different types and sizes of disks, which makes it probably slower, but that's not really an issue when dealing with Time Machine.
Now redundancy is nice to have for a backup - a complete backup for me takes about 16 hours over the network, so I don't want to do this too often, but what's really cool, is the expandability. This means that if I expand the disk capacity in my Mac Pro (which I need to do from time to time as pictures, music and video take more and more space), I can just expand the disk capacity of the Drobo and Time Machine continues to work. When all four disk slots are full, I can begin adding higher capacity disks, as they become available. This works up to 12 TB, which should last - by conservative estimates - for, like, forever (unless I'm getting into producing HD video in feature length)
Since I like to have the backup disk in a different room than the computer, I need a Mac mini in order to enable Time Machine. I happen to have one, but a $599 investment for just making a network backup work is too much. So I really wish for the possibility to us Time Machine directly on a network attached storage - since HP managed do this with it's MediaSmart Server, it should be possible for Drobo to do this, too.
Is it worth $650 (at B&H after mail-in rebate)? Depends on how much you value your digital content and your peace of mind. I value both pretty high, so I'm happy with a solution, where in the worst case (as in technical failures, not catastrophes - that's what mozy is for) only a single hour worth of data (the Time Machine backup interval) could get lost.