Just finished my first month or so with a 2nd generation Drobo. It had a shaky start but it's been reliable since. I guess it's reliable until it's not and then it's sheer panic. What I've learned so far.
It's slow
It's much slower than a conventional USB drive. It appears around 3x slower on xbench and you get around 30MB/sec for an external FW800 drive with 4 SATA drives in it. If you need mega speed then this isn't for you.
Relayout takes a long time
Adding and removing drives requires a length rebuilding process which seems to take between 12 and 18 hours. This is annoying.
How I use it
I keep my iTunes, iPhoto and iMovie libraries on it. They are usable but not super fast like this. Personally, I can live with it. I backup using Timemachine to the unit also. I backup my iMac which is the computer always connected to it using FW800. My MacBook I connect to it occasionaly using USB and time machine it to the Drobo also. You can't connect both at the same time so unplug the FW before plugging in USB.
Alternate backup strategy
You'd be nuts to rely on this as your sole backup. I store all my important data on it as well as time machine backups but I also use jungle disk to back everything up to Amazon S3 (internet storage) in case the unit itself fails. I upgraded to Charter Max for my internet to make this feasible (16MB down, 2Mbit up). I have about 450GB in backups for now. Jungle disk runs in the background and periodically pushes the changes for it's very convenient.
This is getting expensive now. I spent 900 bucks on my Drobo (475 + 3 x 130 for 3 1Tb drives). Amazon S3 costs about 40 bucks a month right now, the fast internet is maybe 30 bucks extra a month so thats 70 bucks a month in total. Thats 840 bucks a year. It's probably cheaper to buy another Drobo than do this but I'd need to keep the other Drobo in work or something. A conventional disk won't work for backups for long as I use HD camcorders which burn up disk space lately (8Gb/hour). That said, I probably could for now, just buy a 1.5Tb external backup drive but this is why I bought a Drobo in the first place. I hated having to buy a new drive every year and the drives seem to last about a year before they would fail (mostly WD external drives).
Interesting info
It seems that Drobo isn't so amazing technology wise. I've been reading that when it's under 50% full then it just uses mirroring and then it switches to RAID-5 when over 50%. I've heard when it uses a single drive (not recommended) then it mirrors sectors on the same drive. This may be why rebuilding takes so long. Restriping from 3 drives to 4 drives isn't cheap.
Am I happy or not
Thats a bigger question :) but for the Drobo, so far, yes. It works although this is the kind of thing where you're happy while it performs perfectly any issues results in zero satisfaction. Would I recommend it to my friends? So far, yes, with provisos on speed expectations etc. A good sign is that their forums are not very busy. There are a couple of complaints but in general people seem not to be complaining about issues there which is probably a healthy sign.