I was looking at this earlier. Some items worth noting:
1) You cant buy an empty case and put your own drives in it (afaik)
This is sort of like buying a ram upgrade from apple; everyone knows they can get the ram cheaper elsewhere.
2) The largest possible array is 7.5TB. Drobo 2.0 offers the capability of 16TB per unit (even though there are no 4TB drives yet). As hard drive sizes increase, this unit is still locked at 7.5TB, unless I've missed something. Also, with a Drobo you can mix drive sizes and models.
3) Its Lacie.
Technically this offers more immediate storage potential than a Drobo if you get the 7.5TB version, but that assumes you are not mirroring the data.
If you dont need true RAID, I'd go with a Drobo personally.
/shilling.