I have been running a D-Link DNS-323 NAS for well over a year now and it's actually been a pleasant experience. I bought it to replace a Netgear SC-101 which basically annoyed the hell out of me because of the need to load special software on each PC wanting to access it, and repeated reliability problems. The beauty of the D-Link device is that you configure it via a web browser and then any PC or Mac on the network can access it as a regular share. So in Windows you simply 'map' a drive and point it to the IP address of the D-Link NAS and the volume name you've given it, couldn't be simpler. Likewise with the Mac you just choose 'connect to server' enter the IP address and volume name and that's it.
At it's simplest, which is how I use it, anyone on the network has read/write access to the whole device and the two 500Gb drives I have installed are combined into a single 1Tb storage space. There are options for combining the drives using RAID-0 (striping) or RAID-1 (mirroring), alternatively you can simply keep your two drives as separate disks. Also, you can define users and groups and use these to control access to anything from the whole device down to individual folders. There are also FTP and Media Sharing services built-in although I personally haven't needed to use these. One further option is a power saving mode which I would recommend (if only to make the device more eco-friendly). After a user defined period of time the drives will spin down if they aren't being accessed, and will spin up automatically the next time you access them. I set the spin down period to 45 minutes which seems a good compromise, so essentially the device is 'dormant' overnight after I've run my daily backup.
My only criticism is that after about 9 months one of the two little LEDs that indicates disk activity, failed but the device still works perfectly. I'm fortunate in that I haven't had any of the reliability problems experienced by other reviewers here, the drive just sits there and does what it does, possibly because I upgraded to the latest firmware. For those feeling a bit more adventurous, there's a DNS-323 community who have developed enhancements and new features for the embedded Linux that this thing uses is it's operating system, although I haven't found the need to go down this route.
As always, any hard disk based device carries a risk of mechanical failure or corruption, for example out of eight Maxtor hard disks I purchased and used in PCs over three years, SIX of them failed. Conversely of all the Samsung drives I've purchased, not one has let me down and I have two 500Gb Samsung drives installed in my D-Link NAS.
All in all I would recommend it based on my experience, it's ease of configuration, (relatively) quiet operation and wide (PC/Mac) compatibility when accessing it across the network.