dbone - don't have the DS1511 to provide a backup solution for the unRAID, but my simple setup provides the functionality of the Synology with the storage capacity of unRAID.
DS-210j (primarily wife's files, Transmission (torrent) machine) 2TB
DS-411 (my stuff/web server/photo station/audio station) 4TB
unRAID (movies, TV shows, music library, photos, backups) 14TB
Various rsync and backup tasks between each server and the laptops. I may use crashplan to backup my movies as that can be added to unRAID. I could get most of what I need from the unRAID server, but the interface is much better through the Synology.
8 unRAID drives nfs mounted to the 210j and -411.
Various unRAID "user shares" nfs mounted to each Synology NAS.
"Movies" unAID user share smb mount to Boxee Box.
Some shares are under the "music" or "photos" drive of the Syno, which allows the use of each Syno App (photo station, audio station). I could also map the drives to the Syno "video" folder to utilize the uPnP server, but not required anymore. I had the movies setup that way when I was using the WDTV Live.
Just to clarify further why I use the Syno functionality instead of unRAID:
Overall: easier for me to troubleshoot if a problem did occur with the unRAID system, and unRAID is a media storage device for me only.
Transmission: integrated as part of Syno, download package for unRAID with a separate console.
I've tested this on unRAID and it works well, but I like to keep my active torrents separate from my movie files.
UPS support: Installed on both the Syno (integrated) and unRAID (apcupsd package download). I could run the unRAID as a slave of the Syno, but I have the unRAID server on a separate UPS.
Audio Server: Integrated on Syno, download package (Subsonic) for unRAID.
Media Server: Integrated on Syno (photos and video), PS3Mediaserver or Twonky can be installed on unRAID. I've never attempted to install on unRAID, well because I don't have much use for it.
File Station/Browser: Ingregrated on Syno, basic functionality on unRAID only - (read/view files), otherwise required to use Windows PC/Explorer.
DynDNS Remote Access: Integrated on Syno with security, unRAID is not secure, but could be set up for Remote Access (with no security). I've tested the DynDNS package with unRAID and it does work well.
rsync: Integrated on syno, script file needs to be added to the unRAID config.
Web Server: integrated on syno, package for unRAID but no security.
Audio Station: Integrated on Syno, requires install of subsonic or similar media player for unRAID.
Network backup: Integrated on Syno, crashplan can be installed on unRAID.
I'm a complete noob with Linux, but unRAID doesn't require in-depth knowledge.