Agreed, read the thread Stephen liked to first.
Bottom line differences are management of array and I/O speeds. Any true RAID 0 or 5 is going to offer faster I/O than Drobo, but usually requires identical drives including hot spares, and needs to be monitored and managed regularly by you. So instead of buying 4 ES.2 Seagates, buy 6 so you can have hot spares. When those are gone, you'll be buying a new set of 6 drives and re-building your entire array. OTOH with Drobo, you can buy your 4 ES.2's now for 3TB net storage and in a year (or two or three) when Drobo fills up or a drive fails, you can add in a new pair 4TB ES6 Seagates for of 6TB net storage; two years later add in another pair of 4TB ES8's for 12TB net storage...
To my way of thinking, the BEST solution is a fast array for working files backed up to a redundant array for security, backed up again to an offsite array for redundant security against direct physical loss. My current strategy -- not necessarily right for anybody else -- is my working files are on a striped array (fast), mirrored to Drobo onsite (smart RAID 5 redundant and okay speed), which is then backed up offsite to older, single drives in a cheap, JBOD fashion, in case of a major loss (older single sata drives are cheaper and faster than BlueRay).