This study looks at the results of using an open textbook in a large enrolment physics course at UBC. The study looks at cost, learning outcomes, use and perceptions related to the open textbook. The authors found that customizing an openly licensed textbook to a course can be “pedagogically useful and appreciated by students, but it may require a good deal of time and resources.”
http://www.irrodl.org/index.php/irrodl/article/view/3006/4220