There is an interesting thread over at Filesharingtalk.com about using a NAS as a media center. As many forum threads do, the discussion gets a bit off topic, but I found one post particularly interesting because it focuses on using a dedicated NAS from one of the major manufacturers such as Netgear, Synology or QNAP vs building your own and using a NAS software solution such as FreeNAS.
A couple of paragraphs from the post sums up the pros of using a dedicated NAS box:
The advantage of running a NAS box from one of the major vendors like QNap, Synology, Netgear or even ASUS with their new ASUStor line of NAS boxes is that they have a very small energy footprint, typically a four bay NAS box will use between 40-70 watts of power, a fraction of that required to run a PC doing the same job. The physical footprint of these NAS boxes is very small too, making them convenient to place out of the way since once running, in theory there is no real reason to go back to the device again.
The dedicated OS that comes with these devices too gives you functionality that you will not find in a PC running FreeNAS, although this functionality varies by vendor, I have configured devices from QNap, Synology and Netgear and find the functionality broadly similar. With cloudbased administration you can even monitor/administer the NAS box from a mobile phone/mobile device and of course access data remotely.
I have personally built a number of NAS boxes using various NAS software distributions such as FreeNAS. I've also owned a bunch of different dedicated NAS servers from major NAS box makers - my opinion is that if you can afford it, go with the made-for-you NAS - probably from Synology.