I've had one for a few months now, just a cheap £50 one (WD MyBook World 1TB) but even for a low-end device it really is brilliant, and it's certainly made me see the value in the higher-end models such as those mentioned above.
The main use for mine is media streaming, previously I'd have to copy stuff to a Passport HDD and connect it to my PS3 to watch in the living room, but now everything is on the NAS. I also use mine for downloads/torrents, as it means I don't have to leave the PC on, plus it causes less disruption to my PCs internet connectivity compared to using an actual download/torrent client on here. Found a good Android app that can control the torrent client so I can even search for stuff and start downloading from my phone.
If you're not sure whether a NAS is for you, I highly recommend just grabbing a cheap one like I did to start with, then upgrade once you've had time to play around. Even the cheap ones can be fiddled with, have new/extra apps installed etc so it's a good starting point.
