Drobo Experience
I have used several models. I am using a drobo pro now. It is connected via the ethernet cable to my macbook (iscsi). The machine is great as you can mix and match drives. I have dual redundancy running.
I have spoken to a lot of people on the drobo. I have never lost or encountered any problems, but never had a hard drive failure when using it. I have added and removed drives with no problems.
Like anything in life there is no guarantee. I use the drobo as my primary drive and have a 2TB drive as a backup to the drobo. (I have less then 2TB of data currently). Could the drobo fail? Corruption is always possible. But I feel safe with the system and my experience using it. Would I buy it again, YES. The simplicity of mixing drive sizes and never powering the unit down is a big plus. Data is always online and accessible. Just follow the rules and procedures from the drobo people when shutting it down.
Now if you are doing heavy duty video processing, you will not beat machine internal storage access with proper drives installed. BUt externally the drobo may appear costly at first glance but when a drive fails how much is that worth and how much will it cost you to attempt to get your data back?
Hope this helps. Check ebay as I have seen some great deals there from legitimate resellers.