If they are good, they are very, very good. A good grief support group allows you to be authentic in your feelings. You can take off your “I'm-pretending-to-be-strong" mask. You are with people who really get what you are going through, and that kind of bond is invaluable. There was a woman in one of my groups who was so sad that her gaze never left the floor. So when she said that she felt that she was not getting what she needed from group, that she felt different from everyone, I asked her to look up and see how when she shared, everyone in group was nodding in agreement. She was missing out on all that validation. Face it. Grieving is a very lonely journey. Don't do it alone.
That being said, a support group needs some essential components for it to be good. The facilitator must be able to monitor time gently so that everyone gets to share. I don't believe in using a timer. There are times when one person may need more time than others depending on the circumstances arising in their life. I discovered my group understands that need and they are willing to give up a few minutes of their time. However, I do not allow group-hoggers. The hogger is someone who is unaware of other's needs. They over-share and take more time than others every meeting. The other group members become resentful. I will speak to the hogger after group and if they are unable to remedy the issue, then I dismiss them. One bad apple can spoil the whole bunch.
Also, if possible, breaking support groups into the smallest common denominator works best. Generic loss groups are not as beneficial in the long run, although short-term they can be okay if the focus is psycho-educational. Putting a widower in with a woman who just lost her baby is not optimum. Yes, loss is loss, but these two have very different grief processes going on.
One more thing—a good support group should not be stiff and formal. It was amazing and wonderful how often my group laughed. I think it's about that shared bond. If your group is depressing you, then you are in the wrong support group. Or maybe it's too soon for you. In fact, I usually do not accept people into group until about three months after their loss because some grievers may be too raw to hear laughter.