I have a Drobo 4-bay, currently filled with WD20EARS drives for 5.44TB of useable space. I started with four 1TB drives and upgraded incrementally as my storage needs increased. Each redistribution of the disk array took around two days to complete, which seemed agonizingly slow to me. I see sustained write speeds of roughly 32MB/s, connected to my Mac mini via FW800. The newer, five-bay S model may see slightly higher speeds.
I use mine solely for media storage, and it houses my movie & TV show collection, as well as my iTunes music library. The device works very well for this purpose, and has no trouble serving up multiple 1080p files to different devices simultaneously. The way I see it, yes, it's slow to move files onto the Drobo, but you only move the files once, so its not that big a deal.
You can use the device for Time Machine backups, though I haven't done so. You would want to create a separate partition for it, otherwise it will eventually chew up all the space on your Drobo. It's perfectly capable of writing to the disk while feeding a 1080p file to one of my other machines, so I'm sure a TM backup would be no sweat.
I bought mine second-hand for a song, so the con of it being expensive didn't apply to me. It is extremely simple to set up, and requires little-to-no monitoring. When the array gets close to capacity (or should there be a drive failure), I have it set to send me an email so I know immediately. All in all, I'm very pleased with it, and would definitely buy another one in the future, even at full price.