It's hard for me to find a single good thing to say about my experience with Drobo products.
First, the FW800 products are S_L_O_W...as in USB2 slow. Will the thunderbolt Drobos also perform below expectations? I would definitely want to see some benchmarks before dropping any cash.
Second, "failure rates" are high and it's a proprietary system so your data is held hostage. Of course, Drobo offers a recovery service. Ka-ching. $300 please. See Scott Kelby's public declaration from a few weeks ago "I'm done with Drobo!" and the 100's of comments generated. Just to be clear here, I'm not talking about a drive failure, but the drobo itself failing. Data on the drives is fine, but you can't access it.
Drobo customer service sucks.
By the time you put some drives in the new 5D, you can buy a 4TB Pegasus for less.
Drobos might be fine for Time Machine BU's, but that's it.