MarlonRC

Lets start by discussing value. The ARM-based pre-build enclosures with 1-4 bays are in most cases at a lower price point than DIY solutions. They are not just cheaper to buy, but cheaper to run: a small ARM-based system consumes a minimal amount of power, runs cool and silent.
But when you want something really high performing with 4 drives or more, teamed gbit connection or even 10 Gbit, you can build it yourself for less. But those solutions aren't really ment for home use.
 
Now lets take a look at the software and features. Personally I prefer the Synology-approach to things over the DIY solutions: The NAS comes with a clean, easy to use web-based GUI and an integrated app-store. While this sounds a litte "apple-like" an somewhat limiting, you can still go inside an tweak a lot. The applications in DSM 6 got to a point, where you have a lot of enterprise-grade features, which are still very easy to configure and maintain. And the last point is the most important for me: I want a NAS, which I can "setup and forget". And at least for me, the self build software solutions never got to that point... Owncloud update for example: have to download the newer version, unpack it, install it and... all data seems to be gone. Then after a little googleing I find a tutorial to migrate the database. An hour later I've got it up and running again, mostly. Seems there is still something wrong with the Apache config...
 
Don't get me wrong, you learn a lot of valuable things this way and that's the reason why I still run my self build setup... But just on a Raspi. For my important stuff, I got a simple and cheap Synology Best money I spent on Hardware in a long time!