Many students dislike being asked to mark other students work and receiving marks from other students. There are many reasons for this, including a belief by students that their peers do not have the expertise to mark reliably. In contrast, students are usually very positive about the benefits of peer review processes where marking is not involved.
PEER Project
PEER (Peer Evaluation in Education Review) Project: website
Prof Ian Hughes, from the University of Leeds, discusses the benefits of using peer assessment in a practical subject in this article in the Times Higher Education.