I have been browsing a site that contains aabout 43 reviews of Moodle which make interesting reading about usability. Apart from the person who thinks the reviews have the "stench of written by the paid for insiders" it is good to see the reaction of others.
Also is it just the teacher in me that causes my teeth to grind when I see people confuse simple and simplistic?
http://www.capterra.com/learning-management-system-software/spotlight/80691/Moodle/Moodle