First thing I would like to comment on is "just reliable back-up of specific files." and you asking about Drobo RAID. RAID is not a BACKUP, RAID protects you against a physical drive failure, it doesn't protect against deletion, corruption, overwrites, etc. Just copying your files to a system with RAID (i.e. Drobo) isn't backing them up, it is just making a copy of them. A true backup solution will let you restore a file to a point in time, reverting changes or restoring it completely if it was deleted.
Now you CAN set this up with an external NAS like the Drobo. For example most of these devices (QNAP, Synology, Buffalo, etc.) have support for TimeMachine (Apple's backup service) on them. I know you said you didn't want OS-system backups but TimeMachine backs up up only a small subset of the OS, it is mainly a user's files and can be configured to ignore directories that you don't care about.
I am currently running a 6 Disk QNAP TS-659 which I have as a TimeMachine target backing up 3 Mac's (1 Mini, 2 MBPs). My QNAP is then running CrashPlan which backs up the other files from configured network shares on the QNAP to their Internet based service.
If you want to add another device to your network you could just back up directly to CrashPlan, they support individual encryption keys so they can't see your data and their unlimited data, 10 computer family plan is less then $200/year. I actually purchased it for my family so I have my parents, in-laws and immediate family all backing up to it.
Another option is the CrashPlan free program which can use an external disk to create backups on (not file copies). Though since you are running Mac's this is duplicating TimeMachines functionality.
Now if you want a Drobo or it's like for more then just actual backups and want to use it as a file share between multiple systems then that is a different story. The original Drobo's and OWC's drive bay's (or other's by caldigital, etc.) require a connection to a host computer through USB, FireWire or now Thunderbolt. Then the unit shows up as an external drive to the system. If it's a RAID unit then the primary benefit of it is that you can configure the disk's RAID to provide striping for fast access or RAID5 for lots of storage (not backups).
Newer units from Drobo and ones from QNAP, Synology, Buffalo, etc. are actual NAS's (Network Attached Storage). They are a standalone unit that attaches to your network (wireless or wired depending on model) and then is configured through a web based GUI. You can create separate user accounts on them so multiple people can store their own files on the system and basically run it as a SOHO File/Print server.