The Drobo is an interesting concept. I got one and sent it back, the file transfers are painfully slow. It has a USB v2.0 interface, but the file transfers weren't nearly as fast as the 32MB/sec of typical USB v2.0 transfers.
From what I can glean from a Google it appears to use a system called ZFS developed by Sun for Solaris. There's an extensive rather technical read on ZFS at Wikipeadia. As John has seen there's many factors to consider given how it works, especially that while it's shuffling data around access can slow to a crawl.
My gut feeling is that the technology might be better suited to large arrays of small, very fast disks i.e. the uber expensive stuff in 19" racks.
According to an (unverified by myself) post on a local forum Drobo recently shutdown their user forum which is not a good sign. This box isn't exactly cheap, does seem to have certain issues and still doesn't solve the most critical need of a serious backup stratergy, having off site backups and it could be a real pain for automated tape backup systems to work with.
I've actually been looking into these as a backup solution. I haven't yet bought one, but it's interesting to see that the photography world is jumping all over them... an example is Scott Kelby of National Association of Photoshop Professionals fame. Their new one is equipped with Firewire 800, so perhaps the speed issue has been addressed.
I doubt that it would be an effective place to capture video (speed), but my need at the moment is to create a safe repository of online storage. I manage several 10TB disk arrays at work and am not keen on having that complexity here at home - not to mention the cost of true RAID.
I'm still not convinced, but most of what I've read from still photographers keeps me interested and watching...
The new Firewire 800 DROBO will be out shortly and should be able to capture video... There are different ways to RAID it. RAID 5 will be the slowest, but I believe you can mirror or stripe the drives as well giving you good performance...
We use Iomega Firewire 800 external drives to capture video from a Canon XH-A1 without a problem...
I originally looked at the Drobo for my backup needs. However, I was put off my the USB / Firewire interfaces - I really wanted a GigE. As such, I settled on the Thecus N5200 PRO. I have 5TB in RAID5 mode. Haven't looked back since.
I had major problems with a Thecus N2100. It appeared that my Seagate 1tb drives were not on the approved list and even though I got them to work in raid1, the box crashed on a power outage and could never see the drives again. Fortunately I was able to get the data off using ext2ifs but decided to go with a D-Link DNS-343. It arrives tomorrow. It's a 4 bay that will do raid0-5.