If you're comfortable building and configuring PCs, I'm not sure there's any reason to buy a pre-fab NAS, but that's a big "if"
If I didn't like fiddling with PCs, I probably would have just shelled out and bought a NAS; even "turnkey" DIY NAS solutions like freenas require some specialized knowledge to work with.
But if you don't mind PC-building/configuration, you can get much better performance for the money just doing it yourself. You can build an x86-64 home fileserver for much less than many NAS boxes cost. As described over
here, I built one earlier this year for around $150 excluding harddrives (with parts on sale). It does everything a NAS does, plus can run a full version of Linux or Windows (mine currently runs Debian). And it can run Media Center on it as a library server which is currently not possible with commercial NAS boxes.