Anyone using most of these cheap/free backup apps needs to make sure they understand their limitations. The main limitation being that most of the cheap/free tools can't backup files that are open.

This means that they won't backup thing like Outlook or Thunderbird data unless your email client is not running (which can be hard to achieve with Outlook sometimes).

They don't all have this limitation, but it is important to be aware whether or not the one you are using does...