Despite the General Medical Council emphasising the value of service users to medical students' education, there is scant literature about service user involvement in medical education. Although some research has outlined the effectiveness of service users as teachers, none has explored social issues surrounding how medical students learn "with" rather than just "about" service users. Incorporating insights from contemporary socio-cultural learning theory, this study examines the views and experiences of 47 stakeholders (comprising 19 service users, 13 medical students and 15 medical educators) concerning service user involvement in medical education. Eight audiotaped focus group discussions were convened and the audiotapes were transcribed. The transcripts and audiotapes were independently analysed by multiple researchers using Framework analysis. Seven content- and five process-orientated themes emerged from the analysis. Content-related themes included the costs and benefits of service user involvement in medical education and process-related themes included the use and function of humour and metaphor. In this paper, we focus primarily on the content-related themes. We discuss these findings in light of the existing service user involvement literature and contemporary socio-cultural learning theory and provide implications for further research and educational practice. We encourage educators to involve service users in medical education but only in a considered way.