My perspective may be different than many others, but thought I would share anyway. I am a psychologist tasked with providing behavior management training to all staff at a residential facility for people with developmental disabilities. My role in regard to that training is content developer, website developer and system administrator. In otherwords, I have insight into all facets of Moodle implementation.
The primary advantage of Moodle, at least in my situation, was that it is open source. There are other open source e-learning systems out there. In my opinion, however, Moodle represents the best of the open source systems in terms of stability, usability and development.
Moodle does have limitations. The interface is rather lackluster and requires considerable effort to do much more than change colors and graphics. Also, it does not provide much in terms of real reports regarding student progress and participation...for either the student or the instructor. I am required to go through several steps in order to import the mysql data into MS Access in order to generate meaningful reports.
In terms of usability, I believe Moodle has achieved a great deal of success. Many of my students have only a high school education and minimal experience with the internet; most have never used an e-learning system before. I have been pleasantly surprised to see that very few students have any difficulty using Moodle to obtain training.
The decision to provide training via an e-learning system was based upon the nature of the services we provide at our facility. Many of our staff have tight schedules that vary greatly based upon the needs of our residents. Trying to tie these employees down to a strict training schedule was not successful. Also, my facility maintains several community residences, some an hour or more away from our main campus. Requiring staff to drive that far and abide by a strict training schedule was unrealistic. E-learning provided a very usable alternative for us.
I hope you find this information helpful!