How well it works depends on how well we use it. I know that I still have a lot to learn in this area, specially in giving instruction in cooperative learning, group management, and student assessment. Effective use of cooperative learning takes time and the results are uneven - just like other instructional techniques. Evaluation of my own implementation is mixed. The 'better' students sometimes dislike cooperative learning because it is less efficient and because they often give more than they get.
Evaluation needs more work.
I'm not sure how we know that we are actually accomplishing what we hope we are with cooperative learning.