Update:
I have the Drobo up and running with 5 WD Red 2 TB Drives in it plus a 64GB msata SSD. So far I have only my music on it, but it's working very well for that. Drives are truly hot-swappable and size doesn't matter (have fun with that, peanut gallery). It was a little finicky during setup and I was a little impatient, which resulted in me losing a lot of movies that someone had kindly shared with me, but I guess that was part of the learning process.
The box itself is smaller than I expected, well made, and easy to use. Lots of bright LEDs, which can be helpful AND annoying. The Drobo Dashboard software is decent but not great, but at least easy to use. One thing I don't like is that in Mountain Lion (or Chupacabra or whatever the fuck we're on now in OSX), it reports the size of the volume, not the actual storage size as configured. So you are basically wedded to the mediocre Drobo software to monitor capacity and usage. Relatively minor annoyance.
I am slightly regretting not getting the 5N (NAS), but this setup in my apartment is really pretty much sufficient as I use my iMac to which it is attached as the Mothership. I am backing up the Drobo and my iMac with using Time Machine with a WD 4TB external drive. I may need a more robust backup solution at some point.