I realize that this is an older question but I feel compelled to add my experience for reference to anyone who may stumble upon this thread in the future.
The Drobo FS is an excellent way to add a file sharing device to a small network but potential buyers should be aware of it's pros and cons when compared to a file server.
''In the interest of full disclosure I will also say that I am an authorized Drobo reseller however people should thoroughly understand what Drobo 'is' and 'is not'.''
Pros
* Drobo maintains it's self: endless Windows patches don't apply here
* Drobo will let you know if something is wrong: easy to read LED lighting on the front panel
* Drobo is redundant: you can have one hard drive crash and keep going (if a drive crashes it will display a blinking red light beside the dead drive - you simply pull the dead drive out and replace with a new one)
* Drobo is scalable: start with two drives and buy more as required
Cons
* Drobo can't take over the role of a Windows Small Business (or any other kind) Server: no mail, DHCP, DNS, VPN, or Active Directory abilities here so if you feel that your network will require any of the above in the future then please look towards a server. (NOTE: Drobo can act as storage if you do decide to scale to a server based solution in the future)
* Drobo does not have any sort of remote access built in: if your people are going to want to remotely access files on your shares you will need to have a VPN solution built into your router or another device on the network (IE: GoToMyPC, VNC, RDP, etc) whereas a server solution would include these capabilities.
* There are cheaper devices on the market than a Drobo (but Drobo FS is still cheaper than a server!)
All in all the Drobo has been a reasonably good device for my own company as well as several of my customers. Of the dozens that we've sold, we've only had one critical failure where a very early Drobo (standard) failed without warning and we lost data.