Exactly, it depends on your:
1. level of of importance
2. cost vs convenience (and how much data you want to secure)
I have about 300GB (or 7yrs of photos, music and audiobooks).
So online backup is too expensive. The library is pretty static, month to month, some new music, photos, a couple of books.
Its just not worth trying to rebuild my library from my DVDs/CDs, friends photos etc. I wanted a mirror of everything, including the OS. With ZERO (well depends on when the last backup was) downtime.
Therefore I kept it simple.. I have bootable backups.
a) RAID 1 (1Tb, ie same size as my desktop) courtesy of OWC's Guardian Maximus, which is a simple hardware raid. Automated SuperDuper to do a smart backup every day. Pretty quick, over FW800 - This is currently connected to my desktop, although later I should do this over my network and keep the RAID 1 in another room.
b) A portable drive that is updated either monthly, after a large session (photos) or just before a major update (service pack/update). This is kept off site.
BTW i wanted a Drobo until i read the small print about it using it's own proprietary RAID.
There's no problem if one of the HDs fails, you just rebuild it, but if Drobo fails... you need to buy another Drobo, or likely something from US Robotics (the makers of Drobo). I would suggest keeping to industry standard RAIDs. For most homes, RAID1 is the simplest.