If like me you're a computer geek, into your computers, media centers and have important files, then a NAS might come in handy. I have recently set mine up, with relativly no experience in anything to do with networking.
I purchased a cheap D- Link 320L from Amazon for about £45. After doing my research and getting some expert advice on setting the whole thing up, I now love having a NAS in my home.
Update 22nd July 2015. Shortly afterwards I purchased a Synology DiskStation DS215j as it's a lot faster, plus the admin area is absolutely amazing, and it also alows me to stream to multiple devices without any problems. It's also number 1 on Amazon under Network Attached Storage!
I have a few websites for clients, a lot of design work and huge amounts of media such as films and TV series, all stored on my computer. Although I use Time Machine on my iMac to back all this data up, I feel as though I need a lot more data storage, and where I can access it from anywhere in my house, on any device without relying on my computer.
A NAS let's you do this.
It's basically a mini computer, housing internal hard drives that you can access from any machine on your local network.
The NAS connects to your router via ethernet, then you can acces it from anywhere in your home.
My iMac is a late 2009 model and only comes with 500G of storage, which isn't a lot in this day and age. With my NAS I bought 2TB 3.5" Desktop internal storage array (x2 drives for RAID1), relatively cheap. It's really easy to install, and if in the near future for example I buy a new iMac I wont have to spend time moving the data from one machine to the other.
One thing I love is playing films on my TV through a media center connected to my NAS without the need for my computer to be on, it's just always available.
Not only is it convenient, but my NAS uses RAID1, meaning it has two hard drives, where if the one fails I wont loose all my data as the second drive will take it's place with all the data still available. So it's also for security, you're not depending on one machine to house all your important infomation. Most NAS's are very energy efficiant, like mine it goes to sleep when it's not being used and hardly uses any power at all.
There are a lot of learning curves when setting up, one big tip would be to connect everything (media center, computer, laptop) via a Gigabit Ethernet connection, rather than WIFI, as WIFI is very slow and unstable, making it almost impossible (in my case at least) to transfer large files. After hard-wiring my computer and media center to my router with an ethernet cable, file transfer is amazingly fast and everything works nicely.