Hi Paul,
Yes, they are old but they just keep on working. I have 4 of them, 2 D-Link DNS-323 and 2 D-Link DNS-320, they are all on 24/7/365 and have been since late 2009. Two are on the network and the other 2 are remote and backup the network pair overnight using rsync.
Since the system has been working without problems why change?