Buy a NAS or build a server?

There is no reason other than you wanting a system where the build is done for you at an insane price markup.

Many $500+ NAS devices will not offer the network storage performance of even a $200 system that you can build that will also have more features (eg more sata ports, PCI-e ports, GPU and connection for an external display, x86 support and support for any current desktop OS).

With all that a system that you build yourself offers, I just cant see what a prebuilt NAS offers. (they are slower, offer fewer features, you cant choose which OS and thus cant make it adapt to new network features (eg for a while, I had an old AMD Duron 900MHz system as a NAS (mainly so I could move from IDE and using a bunch of external (USB 2.0) drive. (other than that, it had great DLNA support, worked well allowing multiple systems to mount it to mount it as a network drive, and could easily hit 100MB/s+

When you build a NAS, it will support the latest standards and will continue to support new standards that may come out later on up until the hardware just becomes incapable of handling new standards in a practical manner.

With a prebuilt NAS, you have to deal with planned obsolescences where a company will release a NAS then stop releasing updates for it a year or 2 later, and since it uses more of a custom OS, you will not be able to really add new features even if the hardware is clearly able to handle it.

I have a NAS I bought from Seagate. It's not all that bad, and yes while somewhat slow, it works. Depends on what you want it to do. Simple backup? Shares? What you want the device to do determines what you'll go with. In my case I just wanted a backup location that allows me to access those files anywhere. I didn't need to mess about with anything complicated. I have something that I can restore files to my system if a crash occurs while at the same time allowing me to access those files from my Mom's or someone else's compy. Just make sure you lock it down so any Joe Shmoe can't get into it.

Nuild your own with Linux or "Free NAS" as the OS.

Building your own system in a desktop case will provide potentially way more storage than most premade NAS enclosures would, and provides easier maintenance/expansion.

If you have the space, then a full tower PC case would be an attractive option, with the likes of the Fractal Design Define XL (http://www.fractal-design.com/?view=product&prod=68) offering a good number of bays.