I'm entirely, completely in love with Drobo as a NAS device.
The ability to pop out a smaller drive and replace it with a larger drive is amazing - that is simply how technology is supposed to work. I have the Drobo FS at home and the DroboPro FS at work. Having used them for about a year and having tried to make them fail before I moved them into production, I'm very happy with their reliability and performance. (More on performance in a second.)
At the high end, I have used EMC and IBM solutions. At the low end, I've used every home-built and crappy RAID NAS solution you can name. Having used three of the five products reviewed by InfoWorld, I can say the Drobo is easily better than most of the units reviewed.
Performance on both the Drobo units I own isn't mind-blowing compared to some of the solutions that cost four or five times more. Ease of management, reliability, price point, expandability and overall functionality far offset the less than awesome performance. Still, as Lifix noted, there is more than enough performance to meet the needs of a home or small office. The only time I really notice the DroboPro FS slowing down is when we're running multiple rsync backups to it.
I have not been this excited and evangelical about a piece of technology since I got my TiVo.
(I'm not in any way compensated by Drobo but would be willing to entertain offers. Drobo? Are you listening? Send me free stuff.)