Patient and Public Involvement (PPI) is a cornerstone of UK National Health Service (NHS) policy. The Cancer Partnership Project (CPP) is the leading national PPI initiative in cancer care. The CPP espouses a "partnership" model, with a "Partnership Group" - collaborative service improvement groups formed of NHS staff and service users - in each of 34 cancer networks in England. These groups aim to enable service users to influence local cancer service development and thereby improve the effectiveness of services. We interviewed 59 cancer service users and NHS staff in a reflective evaluation of CPP. Groups were active and visible in 30 networks, their main activities being: providing an accessible source of consumer opinion; prolific networking and representation; patient information and communication projects; and lobbying for service improvements. The groups exhibited some significant tensions. The motivations of professional staff varied markedly, and "obligatory" involvement as part of a person's job was counter-productive when not coupled with a "personal" belief in the value of PPI. Other controversial areas were the disclosure by patients' of personal health and treatment experiences, and emotional attachment to the group. It was concluded that partnership groups represent a useful PPI model, but more attention generally should be paid to the complexities of PPI and timescales required for meaningful cultural change.