First off, a declaration of (minor) interest - I work in the software bit of a large firm doing storage, including NAS, albeit in network management.
This is a way to have an always-on box to store stuff on, rather than doing the same thing with an expensive PC.
And it seems the only one of the major to do more than Windows or MacOS (i.e. Linux client support) is Buffalo (compared with Maxtor, Seagate and Netgear).