Bought a unit in 2010 and it worked fine for a month or two before failing completely and I lost over 3TB of data. Fortunately, the really critical stuff (about 700GB) was backed up on another HD. Drobo support was pretty much useless and after a couple of weeks of 'troubleshooting' (which included the purchase of another unit), they finally suggested a recovery service that wanted $1200 whether they recovered the data or not.
Bought a Synology 1511 with a 501 and have been perfectly satisfied with this solution for 2 years now.
Basically, if anything fails your data is hosed and the possibility of recovering it is remote because of the proprietary file formats that I don't think even Drobo staff fully understand. Stick with Synology, or QNAP. Performance, reliability and support are far superior, and the peace of mind is priceless.