From the review I've read, if you swap out a drive, it still has to rebuild the array, which could take 10+ hours, same as on any RAID array.
If there is enough empty space before you swapped out, then the drobo array is still accessible (just much slower). If you were tight on space, then you have to wait before you can get to the data.
The advantage is that all this happens automatically, and for what it does, I think drobo is excellent. I run a RAID5 array, and yes, it's a bloody headache for the small-office low-end raid user without tech back-up. If I had a drobo, it would save a lot of hours trying to learn about and setting up the raid, and would probably be worth the investment.
Disadvantages are as you say, no firewire, no network, stupid flashing lights that stay on and distract you.