I have been very positive about QNAPs previous products. Considering the TS409 Pro is near identical to the previous models I used I focused mainly on the changes in this review. Everything I liked before is still there and still works perfectly. The added value comes from the extra disks and RAID levels supported. And when I say RAID I'm not talking about a simple onboard controller, the QNAP TS-409 Pro surprised me with RAID 6 (although slightly pointless with only 4 disks) and features like online expansion and migration. Such functionality is usually only found on advanced controllers you'd find in high end servers.
The TS-409 has nearly everything you could want from it, I'm therefore unsure where QNAP will go with future products. Perhaps an internal switch or access point could be added or perhaps (software) support for some additional USB devices, making the unit even more customizable. I can't think of any huge improvements though, so I think it is safe to say this product - although not the cheapest around - will be a safe investment as it won't require upgrading for years to come. Apart from bigger harddrives of course, which keep dropping in price as well.