I like that you can create courses using your own content, or import from other sources such as Moodle (or another LMS). It also has good integration with Google Apps which makes life easier when trying to set up an automated course flow. The user interface could be improved but this isn't too much of a problem once you get used to it. We are able to use our own content in combination with third party content - we have been able to do this since starting to use Essential Skillz more than 2 years ago now so there's no reason why we couldn't continue doing this indefinitely!