I'm planning on building an unRAID server mainly for home theater use, but will also store my RAWs there. I've looked at all the options, and the main advantage of unRAID is that it takes any combination of drive sizes and types, and if your controller is hosed the data is not - it doesn't use a hardware controller, and the files are intact and readable on the individual drives unlike with most (all?) traditional RAIDs or Drobo.
The QNAP i got user sw raid, so if the box itself dies, I can move the disks to a new box an be back up, or I can boot up my win7 pc with a linux distro on an USB stick and put the disk in that machine and get the files (it have several sata controllers not in use aftyer I got the QNAP). Or I could just pop one of the external backup disks into the PC and keep going until I get a new box.