I bought a QNAP TS-541+ NAS. It came with 8 GB RAM and I added an 8 TB hard drive and a 500 GB SSD.
After downloading the QFinder tool the NAS configured itself and finally showed this Web interface.
The interface is quite snappy and works really really well. In fact I haven't seen a Web interface as well-done as this ever before. (Citrix XenApp comes close but is not nearly as pretty.)
So far I found you can create users and groups, shares (SMB, AFP, and NFS), and an FTP server. The NAS also acts as a Web server and can be a VPN server or even a (apparently Windows-compatible) domain controller.
Each user has a home directory. Only the "admin" user can log in via ssh (or Telnet, if enabled). The su and sudo commands are missing.
The NAS also supports the creation of virtual machines. I think it uses KVM.
There are a lot of apps and features I didn't even try or look at yet.
So far I recommend the device fully.