One class having el ed students doing extra coursework is entirely group work, in order to make a methodological point. The relative effectiveness is very good as the result of freeing up the instructor by encouraging student interaction to solve minor misunderstandings. Usually the students are told that they can always get help on homework assignments. The object is to know the material when they hand in the paper. Homework does not carry much weight in the final grade and the knowledge is tested in frequent quizzes and major tests.
Group testing is very popular.
It seems to me, though, that it has very little impact on grades, either positive or negative. In Elementary Statistics I have a component consisting of four small projects, including in-class oral presentations. These are supposed to be group efforts, but it often seems to be the presenter who does the work. Using cooperative learning on quizzes and exams has been quite successful this semester. Students take an individual exam first, submit it, then retake it as a group.