I have a study group and it works for me. At the begining of the year, our group works on outlines and towards finals we do pratice exams. This seems to work well for our group. Our group has the #1, 5, 8, 12 rankings.
However, study groups do have a tendency of getting off subject. Our group has a rule that we get our studying done and then we go have lunch or get drinks after we are completed.
Like most things in law school, you have to find out what works good for you. If working in a study group works for you, stick with it. If studying on your own works for you, do that.