I used a Ximeta NetDisk for several years. Still have it but use it USB now. It was great when I had an all Windows network, but when I got a Mac I found out I would have to reformat it to FAT32 and then I still would only be able to assign write privileges to one computer at a time. It was nice, though, because it worked like a local drive--you didn't have to map anything, but you did have to install a driver on any computer that was going to use it.
Since adding a Mac to my network, I switched to a WD NetCenter. It works just as well as the Netdisk, but it's noticeably slower. Fast enough for music, but video is choppy if I try to run it off the NetCenter. This one also acts as a print server (I don't use that feature) or you can plug 2 more USB HD's to add them to the network. If the USB drive is formatted FAT32, it becomes read/write, NTFS is read-only.