A major factor in deciding what to go for is whether it is for home or bussiness use. I cant comment on bussiness use, but if you want a NAS (network attached storage) device for home use, then QNAP do some seriously powerful bits of kit. I bought a QNAP TS-109 Pro and then bought a 1TB disk separately and fitted myself. Very easy to do, and network setup is easy for anyone with just a little home networking experience. The main reason I went with QNAP was that it was DLNA compatible, which means it can also be used as a media server for stuff like the PS3. It has many features like web server, FTP host, itunes server, bittorrent download station(even without a PC turned on), backup server, remote access over the internet. Highly recommended.
PS: I DONT work as an agent for QNAP, I am just well impressed with what I got from them :D