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.
You can beat them easily for less money if you have even modest technical know-how. FreeNAS/NAS4Free have web interfaces. They are a little trickier to set up and use than some prebuilts, but also gives you ZFS and ECC memory (if your mobo supports it). ZFS is key. Without it, even the top pre-builts like Syno's are using ext3/4 which do not have block level checksums. This increases the likelihood of errors like bit-rot aka silent data corruption. Did I mention they are cheaper than prebuilts, and due to software RAID they can use any type of hard drive? You can use non-Red WDs.
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.
75 watts? Er, if you use antique hardware maybe. My NAS idles at about 35W when idle, with six WD Red 3TB and Hitachi 5-platter 3TB hard drives. If they were all Reds and if I used less or lower-voltage RAM, maybe less. This is with 16GB ECC memory and a Pentium G530 CPU (which is ECC-compatible) and USB stick for boot.
How much does your Syno idle at? Even if it idled at 10 W, and you kept your NAS on 24/7/365 that 25W difference adds up to what, $22 per year? That's more than offset by the much lower initial cost (and possibly lower HDD cost if it allows you to use HDDs that would not be suitable for use with hardware RAID), and that's not even including the benefit of ZFS.
Don't get me wrong, Syno is great for those who value ease of use very highly, but you do pay more and get less data protection, and the small savings from energy efficiency aren't going to offset the initial cost that much assuming you are using up to date, energy efficient hardware and not some old beater Pentium 4 with no-name PSU or something.