Group projects are very beneficial. Group tasks during the session are quite effective. Group summary tasks or quizzes work well. You can give harder problems to solve. They learn more explaining the solution to the group.
Students really do help each other when given the proper incentive.
The projects really help to connect the course with the world. Tests in groups are particularly good at reducing test anxiety. Projects are ideal times for group learning, as time-consuming tasks can be more palatable with several people to do the work. I think these are all successful in the sense that students learn about group dynamics and how to play their proper role in a group situation. I find these activities successful.