I've owned a 2nd-gen Drobo and now a DroboPro. My experience with it is very mixed. My opinion is that it's a good solution for someone who can't be bothered with mdadm anymore and needs a lot of storage, with some important caveats.

When it works, it's fantastic. When things start to go wrong, there's nothing you can do about it except experiment with switching drives or email support and hope they can diagnose it.

I loaded my first Drobo with Seagate 1.5tb drives. It constantly dropped drives out of the array and was very slow (<10MB/sec over FW800). After several back and forths with tech support I gave up in frustration and didn't use it for a while. Then 2TB drives became available, I swapped them out, and it stopped dropping out.

I then upgraded to a DroboPro as I had outgrown the 4 drive version, which was painless. It was still slow, but not as much. The log is now ENCRYPTED(!!!) so there is absolutely nothing you can see for yourself other than whether a drive is "bad" or "good". If you suspect that one of the drives might be failing, you have to email your encrypted log to Drobo and wait for them to respond. Which is pretty frustrating when you're in another timezone and each exchange takes 24 hours.

Somewhere among switching around drives and upgrading firmware it became faster, and now to me seems acceptably fast. I think I must have just had a bad drive or three, but of course without access to any logs it's a tedious and expensive process of trial and error to figure out.

All was well, and then just shy of a year it stopped booting. I bought it in one country and then moved, so had to ship it back to the original distributor, which is annoying and expensive but understandable, but of course you can only access the drives with another Drobo, and means I'll be without access to my stuff for about a month (except for backups). I wasn't too impressed about what is marketed as a professional product designed to run 24/7 failing after less than a year of light use.

Pros:

* Very easy to use. Auto-expansion is fantastic and works as advertised. You literally only have to shove a new drive in the slot and wait a few minutes.
* Don't have to rebuild when you add capacity
* Don't lose all your data if you disconnect 2 drives at once, put them back in a different order, or any of the myriad interesting ways you can accidentally kill your entire array with RAID.

Cons:

* Somewhat pricey. But you get what you pay for - at least I did until it died.
* Encrypting the log files is outrageous. This is probably the worst thing about it.
* No transparency about what it's doing. It's quite literally a black box.
* iSCSI on Mac doesn't reliably reconnect if you unmount it. I usually have to restart the device and/or my PC.
* No way of accessing data (other than another Drobo) if the device fails.
* Not particularly fast. Much slower than a bare drive. 40MB/sec if I'm lucky. No idea how they got VMware certification; trying to boot from it would be agonizing.

Other thoughts:

* Others have said it sounds like a jet engine. It's not quiet, but it's by no means loud. It's just two 80mm fans spinning at maybe 1200rpm or less. Drives always seem cool to the touch. You probably couldn't sleep with one next to your head but in the next room, no problem.
* It takes days to rebuild 8TB when you switch out a bare drive. So does normal RAID though.