I have used many. I thought that the Buffalo was the best until I got a Synology. I have a 212+ and a 712+ with WD Red drives in them. Can't beat these NAS units. Yes they are a bit pricy but when I stopped to figure what I have on these drives it's well worth the expense.
I bought a low end Synology (211j) and then quickly grew out of it. Then I considered building my own, but instead just bought a higher end Synology (DS413).
Building my own NAS would save maybe $250 up front, but it'd be less user friendly, and use maybe 75 Watts more. Going by the rule of thumb that every Watt equals $1 per year, we're looking at $75 more per year in electricity.
I now use the DS413 as my main NAS (many Terabytes), and backup over the network automatically to the 211j daily.
But my Synology DS213 is sexy as hell 😎At some point, I would rather build my own NAS that paying through the nose for some of these...