When I was first diagnosed, I joined a support group at Gilda's Club. While I appreciated being with people who could understand what I was going through, because I had such a rare cancer (Appendix Cancer), with an unconventional treatment (Intraperitoneal Chemo) there was no one for me to really share with. Most of the others in the group were breast cancer or lymphoma/leukemia. Thus, I felt like the "odd bird." In on-line support groups without geographic boundaries, if you have a rare cancer you are more likely to find someone in a similar situation.
Both types of groups have their pluses and minuses.