I'm a big fan of Synology NAS boxes for three reasons:
1) Open RAID, unlike Drobo and Netgear ReadyNAS. Under the hood it is all just Linux md and lvm. If the NAS dies, I can mount the drives on a PC and recover the data.
2) Open OS. Again, Linux. I have root, and I can make it do whatever I want. One of my 1TB drives died and I replaced it with a 2TB drive. The RAID re-mirrored to the new drive as expected, but what to do with the extra 1TB of space? Some quick magic in the shell, and I have a second non-mirrored 1TB volume for Time Machine. Best of all, the GUI picked up the changes and lets me manage the second volume, even though my model doesn't officially support this configuration. Synology's DSM software and add-on packages ain't too shabby either.
3) Overall great numbers. Even tho my DS212j is rather low-end, I still don't think I can home-build for the same price and get anywhere near the same performance in a small, power-saving box.