A big problem, if you're diagnosed with a serious or chronic illness or a loved one is diagnosed, is getting past the isolation to educate yourself, learn coping skills, and eventually, formulate a life that includes (but isn't defined by) illness. Good support groups are like the running water in this photo of a brook in winter. The water doesn't change the reality of the cold or the ice on the branch, but its motion, its presence, reminds us that the branch isn't alone in the cold... life and life's activity is still there.
The goals, structure, and pros and cons of support groups vary depending on the medical condition that brings people together, whether the group participants are the patients themselves or friends or family, and how good the group moderator/facilitator is.
If you are inquiring about a support group, you want to make sure you find the group that is most appropriate for you. For instance, get specific information about the following for possible groups: who runs it, the condition around which it is based (if it is cancer, for instance, is it a certain type of cancer, cancer in remission, incurable cancer, etc.), participants (patients, family members, spouses or significant others, children, or nonspecified), and the background and experience of the moderator, both in terms of ability to run a support group of people who are there by choice and whether they have direct experience with the condition in question.
I have participated in three support groups over the years: one for people with epilepsy who were working or had been employed, one for couples in which one member had epilepsy, and one for parents of children (of any age) who had a mental illness. All three were very helpful, and each was very distinct.
The first two were run by the state epilepsy foundation, were held in a city from which many possible appropriate participants could be drawn, and were moderated by an experienced social worker who had epilepsy herself (the same person, actually). The groups were focused in their makeup and goals, and the moderator was very good at enabling people to vent, rage or weep, but know when to gently stop someone so someone else could respond or have a turn. The moderator was like a good school teacher: Everyone had a chance, no one hogged all the time, and each person was kept on topic and polite in language. The moderator was gifted at seeing when a person's line of thought was going somewhere intensely personal and having them pause to see whether they wanted to continue or not.
The couples' and parents' groups differed because at least some of the participants did not have first-hand knowledge of the condition at hand. Indeed, the parent group had a very broad common base, mental illness, and the moderator was quick to point out that every person's inputs had value but one person's experience might have little or no implications for another family's experience or possible responses to a problem. On the other hand, the parents' group was the most polite, with people quick to empathize over someone's problems either medically managing a child's problems, handling the family/social/educational aspects of the illness, or accepting their personal limitations, and (I thought) very respectful that what was said at group was said in confidence.
Having noted all those things, I would not personally say anything in any group that I couldn't accept being repeated and attributed to me (or my family). In my case, because I had been a doctor before I had a head injury and developed a seizure disorder, I also had to avoid giving medical advice but not keep silent when someone said something that was factually wrong. I learned a lot of diplomacy in group, including the ability to see and accept when someone wanted help and when someone didn't really want help or advice, but simply the chance to vent their feelings without fear.
I am a firm believer that support groups can be immensely helpful at filling needs that medical care itself cannot fill, but only if they are well run, appropriate for your personal situation, and you are willing to make the commitment to show up, share honestly, listen thoughtfully, and respond thoughtfully.