(Note: This is cut-and-paste and then edited from my answer to a similar question on apple.stackexchange.com.)
The Drobo FS is my new favourite NAS. I've run a Netgear ReadyNAS and a PC-based NAS in the past and neither touch the Drobo FS for simplicity, speed and reliability. The ReadyNAS suffered from poor RAID performance, especially on reboots when volumes needed to be scanned. And the power supply in it was really lame. And the PC-based NAS was just a pain to maintain and costly to keep up.
The Drobo FS is super simple to set up. Has very, very good volume build and scanning speeds on startup. Has what has to be the best hot-swapping of any multi-disk unit I've ever seen that was in the sub-$10k range. And performs like a champ while drawing very little power.
They appear a little pricey out of the gate, but you save much money in time and you don't have to populate it with drives purchased from Drobo. You also don't have to take care to match drive sizes like many NAS solutions require for maximum performance. For example: I'm currently running two 400 GB and two 1 GB drives my older Drobo -- something I could never do with my ReadyNAS box without it pitching a fit about mis-matched drives.
I now run an FS for home sharing and an old Drobo hangs off my iMac for audio project data. They are teh awesome.