Before, I found this extension, I was reluctant to make any drastic changes to my sites. However, the ease with which sites can be backed up and restored has set me free to make improvements. I encountered a bug with some versions of PHP4, however it works flawlessly on PHP5. The developers are very active on the support forums and its worth sticking with it.
Finally, it is so simple and straightforward to use, I can't believe it's free.