I belonged to Toastmasters from about 1999-2005, but the membership at my chapter dwindled so it disbanded, and then there was Katrina and people had too much on their plates as it was. Anyway, it has since re-started, I think, but I haven't been back.
It is DEFINITELY worthwhile if public speaking ability is important to your life for some reason. It helped me at work.
One of the little exercises is to talk for a few minutes extemporaneously on a topic that is given to you and which you may know nothing about. That was tremendously helpful to me at work, in meetings where people ask you questions out of left field. I learned that people are very complimentary and happy if you just talk around the subject while conveying that you are at ease and confident, even if you don't know a thing. (When I found that out, my opinion of the value of meetings went even lower.)
I think that the practice in brief, extemporaneous speaking was more helpful to me than the speeches I did, frankly, because giving a predetermined formal speech is easier to me than speaking off the top of my head.