The QNAP QBack-25 is one awesome HDD enclosure. Its 2.5" form factor allows you to use notebook harddisks which consume very little power, so an external power supply is not needed. Also the smaller size makes the unit more portable than a big 3.5" enclosure. On the other hand you may have to buy a HDD just to use this unit because you can't use your old 3.5" HDDs.
USB performance is good. With speeds of above 30 MB/s an installed 2.5" HDD might become the limiting factor anyway. What makes the QBack-25 stand out over all other HDD enclosures I used so far is the excellent software. It comes with a ton of features, yet its use is intuitive and can be done even by less experienced users. The feature which lets the software automatically find important folders like My Documents, Fonts and Outlook E-Mails helps a lot if you don't know exactly where those files are stored. Since backups can be done incrementally your backup times are generally much shorter than when you just copy all files over for every backup. Being able to access the backup data without the QBACK software in case of an emergency is a great feature that even some professional backup solutions are missing.
Should you have important data (pron?) to hide (from your mom/wife/sister?) an optional 256-bit AES encryption should keep your data safe from prying eyes. Actually the US government recommends this encryption for "Top Secret" data.
The build quality of the QBack-25 is great, its metal cover also adds to the aesthetic impression of the unit. With the unit comes a little protective bag which helps protect the unit from scratches during transport. Overall I am very impressed by the QBack-25, QNAP has done a great job in delivering a rock solid product, which also comes at a very price.