I have had the FireWire Drobo for a short while now, and love it. I have had no problems. I started with 3 bays full, and when it warned me about reaching capacity (90%) I plugged in another 320GB drive into the 4th bay. The capacity was immediately available, and I had no problems with data loss or anything.
If I were asked what I like/don't like about it:
Like: Simple to set up, easy to add additional capacity (and you don't have to reformat to do it), can use bare internal SATA drives
Dislike: It is a bit noisy, and is slower than a traditional drive
I considered a RAID system, but then realized that anytime I wanted to add capacity I would have to reformat a new array. That would mean copying the data from the old array somewhere, setting up the new RAID, and then copying it back. No thanks.
Oh, and I use my Drobo for external storage of my iPhoto and iTunes libraries. I have an additional 500GB external USB drive that I use exclusively for Time Machine. That way if my internal HD fails or a Drobo drive fails, my photos/music/videos are fine. If I accidentally delete a file(s), I can use the Time Machine to restore. The only thing I am not covered for is a disaster to the house that would wipe out all drives. For that, I think an additional drive in a safe deposit box would suffice.