I've been in Toastmasters since 2004, earning an AC-Gold and oCL.
I originally joined because I often find myself speaking outside Toastmasters and wanted to do a better job. I was initially skeptical that 7-minute speeches would help much toward one-hour and longer technical presentations. I was wrong: there's no question that it's helped me a great deal.
The key is getting lots of practice in how to communicate ideas. It helps to belong to a good small club that gives you lots of speaking opportunities. My home club is Coherent Communicators, where I'm VP of Education. If you're in Silicon Valley, come check us out! We always welcome guests and prospective new members. I'm also a member of GeoSpeakers, a terrific and friendly club at the USGS in Menlo Park.
Finishing up your CC and wondering which manual to try next? Read my admittedly biased Review of Toastmasters manuals.
I get annoyed by secret, unwritten rules, so I'm compiling a list of Toastmasters Secrets: The Unwritten Rules.