I am working on a site where I want to use an Open Source CMS. There is a great resource to do research on this exact subject at Open Source CMS. It creates a fresh install of lots of different CMS systems every two hours.
So, I used Drupal for a minute because I had heard good things, and it comes highly rated at Open Source CMS. Druapl truned out to not be what I needed for this project. It is very powerful, but it was not very user-friendly and intuitive, at least to me, so I figured this would not be good for the less-technical users of the CMS.