Yes I discovered AUFS a while ago and it is really great.
I use it to backup to 5 disks (of various sizes).
Need to backup 10TB to a bunch of disks, but in the case of a disaster I want to be able to read individual disks without setting up a (software) raid array when restoring from an emergency. So I joined 5 disks of various sizes, 10TB in total, together with aufs and write to the aufs device. Aufs ensures that files are written on one of the disks, the one with the most space left.
Later I can take an individual disk and find part of all files on int, or put them together in an aufs-setup and restore in one go.
Raid-JBOD has the drawback that loss of one disk (in the backup set) means loss of all of them.
Raid-5 is more complicated and fragile for restores, and wastes 1 disk for parity, which is not required for a backup (the live system already is raid-6).
(and yes, I've got two backups).