I agree with BSA - and although I've read some good things about the drobo boxes - I've also heard some nightmares as well.
If you're even slightly handy with PC hardware - you can put the money into some hardware and have the same performance, with a little bit better reliability (albeit with an end result that's not nearly as attractive - looking that is).
I've built a few NAS servers using OpenFiler and a couple of other opensource solutions. Provided that you spend a little bit of time researching compatible RAID cards, everything else can be whatever is cheapest at the time. Most of them have pretty active forums for support and whitebox server HCLs which make the purchasing a no-brainer as far as compatibility.
If building your own is not agreeable to you, then the Drobo or similar NAS unit will work just fine. If you don't mind rolling your own... check here:
OpenFiler - free for most uses - cheap add-on modules for more advanced uses.
FreeNAS - a little less user-friendly in the setup - but works great also. You can read through tutorial
here and
here
There are others, but those are definitely the two 'big dogs' on the subject.
I've also had good luck with a few of the hardware only solutions from Promise and Netgear as well as the DroboPro - however, I've read of problems will all of them as well. The nice thing (for me at least) about building my own is that I can maintain it better down the road - and I don't mind having a full tower running in my closet. For you the answer might be a different one however, so these are just a couple of other options for you to weigh -
not a recommendation
against Drobo - I'm sure one of them would meet your needs nicely as well (albeit at perhaps a slightly higher cost).