Moodle is an incredibly well done and well supported product. It came in at the top among our choices which included both commercial and open-source offerings.

Installation is straight-forward. You must be server-savvy.

The administrative, instructor and student interfaces are very clean and relatively clear so no additional documentation or training is required.

Excellent set of additional customizations (activities, modules) provided at no cost by 3rd parties. Note that the basic moodle distribution is so complete that these are not necessary, merely extensions of value.

Upgrading from one version to another is easy, but any customizations must be dealt with as you'd expect with this kind of system and architecture.