This article reports on a large scale implementation of personal response units in three introductory science courses at the University of Western Ontario in Canada. An online survey of students was conducted to gather their perceptions on the uses of the devices, triangulated by participant observation of the classes and email interviews with the instructors. Although the students' perceptions were generally favourable, problems associated with implementation were widespread. Advantages and disadvantages of the technology are discussed along with suggestions for its use. (Contains 5 figures and 1 table.)