Yup, this is basically a commercially produced version of that box.
Except the Debian box is more customizable by the user. The DNS-321 is limited
to the specific features that D-Link chose to implement. For example, with respect
to Samba (and this may be relevant to your multi-user requirement) the DNS-321
samba server is setup with security mode = SHARE. This means you can only assign
a single password to a share. See http://www.samba.org/samba/docs/man/Samba-HOWTO-Collection/ServerType.html for more information.
The more flexible security mode = USER is not available (without voiding your warranty).
Look for the line 'security =' in the /etc/samba/smb.conf file on your Debian Box to see
which mode is currently working for you now.