Today I had the chance to test a NAS that certainly doesn't address the average user but the enthusiasts and small to medium sized companies that seek a highly reliable and feature-rich Network Attached Storage solution that will fully cover their needs. So it is natural that the TS-669 Pro comes at a high price, since it has a lot to offer and on top of that it is supported by QNAP with constant firmware updates and their active official forum.
Besides the high speeds that the TS-669 Pro managed to achieve, although I didn't manage to get LACP working from the client side, even with a managed switch, I was left very satisfied by its reliability and the low noise. Normally equipment that is intended for bussiness environments doesn't feature quiet operation but apparently in this case this is not true. Also QNAP's operating system proved to be rock stable and never crashed or even stalled during all the tests I conducted, which lasted many days during which I had the NAS running at heavy load continuously (24/7).
The only problem with devices that support so many HDDs, is the high price of drives. But if you can afford the TS-669 Pro then most likely you can afford six HDDs too, or you would buy the smaller TS-469 Pro.

To sum up, if you need a lot of storage and features, and you can pay the high price then go for it and you will be left very satisfied. It needs no Linux knowledge, you can set it up in a few minutes using its intuitive interface and the network speeds it achieves are very good. I also recommend to check QNAP's official HDD compability list, so you have the best drives for your NAS and pair it with a good switch to maximize its potential.