I own a Drobo Mini.
1. Yes, it is $650 with no disks; but that's how all Drobo devices are, there are some that are up to $16,000 without disks. Drobo is not just about redundancy, their Beyond RAID technology is fantastic. The amount of customization you can do with Read/Write is astonishing. Also, the Drobo Dashboard app is great, it allows you complete control of the array with an easy to use UI that almost anyone can figure out, even if they have no experience with a computer.
2. Compared to having two drives, the BIGGEST perk of ANY Drobo is hot-swappable drives. If one goes down, you won't lose your data and you can pop the drive out and throw another one right in and it automatically writes the data to it, assuming you have the array set up as Write-Duplicate. It is a lot more convenient than carrying two drives around, I've currently got 3 LaCie D2's sitting around and now this Drobo.
3. It's a fast little thing. Faster than my three LaCie D2's, and that's only using USB2.0 as well. I've got 750GB 7500RPM drives in mine. If I could afford to replace my 5 year old computer with something with Thunderbolt, this thing would smoke.
5. It is NOT as big as a MacBook Air. This thing is TINY. It's 1.5 inches high, 7.5 inches wide, and 7 inches deep. Half the size of a MacBook Air. It is made small for ease of transportation. When I bring my video rig on shoots with me, I'm able to fit my entire camera rig, including shoulder mount, 13" MacBook, Drobo Mini, iPad, and almost ALL of my camera accessories in one backpack.
6. This was the smartest tech purchase I have EVER made.