I recently bought Dlink DNS-323. I am using this in RAID 1 mode to copy my data and archive images.
I am also using it as media server (UPnP) for streaming video / audio and image content to PS3. The whole setup is very easy and lot of fun! I copied entire catalog of movies / audio and digital image archive and realized the setup is not working properly. I kept getting error while movie streaming on PS3.
"This content cannot be played (800288D8)"
I was very frustrated by the error message. Earlier I thought this is happening due to lack of processing power at DNS-323 end. After careful examination that possibility faded. DNS-323 has Marvell 500 Mhz processor, 64 Mb RAM, 1000 Mbps Ethernet connectivity. There are two Samsung Spinpoint 1 TB each HDD placed inside. I wasn't expecting any bottlenecks for couple of users.
I was about to return the unit and then I came across this fantastic online resource. I installed Mediatomb as UPnP server and disabled in-built application. This allowed me to have seamless access to my video content without any issues! I will recommend you to install this on your DNS-323 so that you can enjoy secure storage and media server. 
This setup is good for -
- Archive of all of your files. No more nightmares of hard-drive failure!
- Cheap and abundant storage.
- Media serving capabilities at home.
- Expose firewall-enabled FTP server over internet.
Don't expect miraculous speeds though. 3-4.5 MBps is average transfer speeds for me. This is enough for me to backup and access stored data. For faster access, I prefer firewire or USB 2.0 drive. Figure out yourself about your requirements. Ciao!