All in all the DS412+ left me satisfied with its performance, high reliability and especially its nice operating system, which is very easy to use even for inexperienced users. The NAS proved to be astonishingly fast even in RAID 1 mode and surpassed competing QNAP boxes with similar hardware. However the DS412+ lost the battle of the external storage tests due to its picky USB 3.0 ports. The external USB 3.0 disk we use for our tests wasn't compatible and operated in USB 2.0 mode so the performance hit was huge. Hopefully Synology will look into this matter and try to fix the compatibility issues. Meanwhile you better use the eSATA port for all external drives in order to avoid such problems or acquire one of the USB 3.0 external drives that Synology verifies as compatible.
Synology had to make some compromises in order to lower the price of the DS412+ as much as possible. The most noticeable effect of this are the plastic case and the plastic HDD caddies. It would be nicer if they had used a more rugged material, but on the other hand it is better to make compromises in cosmetic areas and not reduce hardware cost at the price of performance. While the available RAM is enough for a Linux box it would be nice if it could be upgraded easier, since opening the Synology DS412+ is not the easiest thing in the world and on top of that there is only a single SO-DIMM slot.

To sum up, the Synology DS412+ is a highly reliable speed demon, with a ton of features. It is offered at a fair price, which is much lower than the competition, so I have no problem recommending this product to anyone interested in a high performance 4-bay NAS.