I have used three Drobos. An original 4-bay Drobo attached via USB2, original Drobo Pro (8-bay) attached with iSCSI, and I currently use a Drobo S (5-bay) attached via FW800.

On the 4-bay, I once ran into a problem expanding the volume. I believe I was going from four 750GB disks to four 1TB disks. The first three 1TB went through relayout fine. The last one couldn't for some reason. Drobo couldn't figure it out either, and they sent me another 4-bay unit to try with my existing disk pack. They did this a few months outside of my original warranty, which I think was a nice gesture. In the end, I decided to keep my original unit since I did a fan mod on it. I didn't lose any data since three of the four drives were always working. I moved it all off to external hard drives, built the 4x1TB volume from scratch, and then put the data back on. After that debacle, I upgraded that same drobo months later to 4x2TB disks without any issue.

We used a Drobo Pro with dual disk redundancy at work for a couple years. It hosted backups, disk images, and all kinds of miscellaneous stuff. It was connected via 1Gbps iSCSI, but it was slow as hell. We still have this unit in the closet since its firmware is stuck in a time warp. Again, Drobo support can't figure out what's wrong. It simply can't have its firmware (for larger HD support) upgraded through any means. It still works, you just can't put in disks with 4K sectors until the firmware situation is worked out. Lame.

At my house, I currently use a Drobo S hooked up via FW800 to a Mac Mini. This is my file server which contains five 3TB drives (single disk redundancy). By far, this is the fastest Drobo I've owned. It's still nowhere near as fast as a single SATA drive, but copying a file to it doesn't make you want to cry. The Drobo S was originally populated with 2TB disks. The upgrade to 3TB disks happened without any drama.