I’m guessing from the amount of hits on the Drobo article from 2009 that people are still having problems with Drobos rebuilding the array in a decent amount of time.
Ever since I got a DS4600 using standard RAID-5 I’ve been quite happy. Rebuild times on a 6TB volume are about 2.5 hours. Note: the volume is only about 1/3rd full, but it’s still way more data than what was on the Drobo in 2009.