I have retired the pc I used for a NAS. The reason why is because it didn't make a whole lot of sense.
I found it made more sense to just use the pc I already use every day in the home. File serving and streaming in the home isn't very intensive. If you already have a good pc that you use then adding more drives and letting it do the work makes a lot of sense. I know people say that they want to put it separate because it uses less power with something like an Atom cpu, but it isn't a huge difference.
A normal pc that is sitting idle will put the cpu into low power states, my video card automatically under clocks when not in use, and drives can be set to power down when not used so it really is pretty close to a standalone unit. Something like a core2 cpu has more than enough idle time even when running applications to send out a file to another device in the home.
You already have all the other applications you use for media and online tools for torrents, usenet or whatever so it is really convenient.