We've been running an OpenFiler box for about 8 months now, mainly as a backup for the stuff that won't fit onto tape any more. We've got it running on an HP DX2200 workstation with 2 x 1Tb mirrored drives. Its been absolutely brilliant, the only limit being the number of hard drives we can fit in the box!
I've also just ordered a DroboPro and 8 x 1.5Tb HDDs - came to around £1500 - far cheaper than looking into new tape technology (as richardp mentions) and about the same cost as building a new Openfiler box. This should give us 9.36Tb usable capacity with single-disk redunancy or 8.17Tb with dual.
I believe the Drobo's can also be chained together using "Smart Volumes" to give a maximum of 16 units, each with the potential of 16Tb capacity.
I don't think I'd consider it as a replacement to an Enterprise-class storage device, but as a backup medium its looking very promising.