I've had my DS411j for a few days now and I absolutely love it. It's the third NAS I've owned and is much, much quieter than the previous two (a Buffalo Linkstation and a Thecus N1200). I installed three 2TB Samsung HD204UI drives in it and it took about 14 hours to build the RAID5 array, which gave me plenty of time to read the extensive help files in the device's very slick web interface.
The media server is also much better than those on my previous two NAS devices, including the Twonky server I installed on the Thecus. If you're planning to use this to serve media to a DMA (digital media adapter) it'll be perfect.
Transfer speeds aren't up there with the high-end expensive models that are targeted as businesses but that's why this thing is £265. I just did a test transferring a 600MB file from my Windows 7 PC to the DS411j and back again and network utilisation was about 25% of my gigabit connection in both directions, with jumbo frames enabled. So it's more than fast enough for serving high-def media and anything else a home user is likely to do with it.
There are plenty of clever things you can do with it that I haven't tried out yet such as installing PHP/MySQL and content managers like Drupal, or creating iSCSI targets so that network PCs can use parts of it as native hard drives (including booting from it if you're a masochist).
Here's a tip for Windows users - if you want everyone on the network to be able to seamlessly access files on the DS411j without being prompted for login details, make sure you turn the guest account on (and grant it privileges to the shared folders you want to be accessible). It took me a while to figure that one out.
So, this is a well-built, quiet NAS with vast capacity and some resilience from disk failure (if you set it up that way). For what you get, the price is amazing.