Haptic assistance seems to be a very promising option to reduce the complexity of a surgical control task while keeping the surgeon in-the-loop and allowing intervention at any time during surgery. Based on experimental set-ups enabling the interactive simulation of different modes of feedback and levels of arbitration we perform user studies regarding different assistance functions to evaluate their effect on system usability. The aim is to identify appropriate configurations depending on the requirements of specific intraoperative scenarios as a basis for a comprehensive framework and modular user interfaces providing a flexible integration of cooperative robotic functionalities suited to the needs of a particular surgical scenario.