I'm using 2 of the 2tb green drives, and 2 older 1.5 tb units scavenged from other external hard drives with my mac mini server and couldn't be happier.
Of course, I could have without the one screw up where I tugged on the firewire cable and sent the drobo crashing six feet onto a concrete floor. The drives still perform well, and my data was perfectly intact though my original drobo was smashed like a car in a high speed collision. I just slipped the hard drives into a new drobo, and moved on.
It was the second failure I've recovered relatively painlessly from, (the first was an older recycled hard drive that failed) I am completely satisfied with the Drobo unit's performance.