Just met a guy in the airport who worked for the company that makes the millimeter-wave full body scanners. His opinion? "You couldn't pay me to get in those things."
According to him, initial tests on safety were "inconclusive", and subsequent tests haven't been much better. With most health effects likely years away "inconclusive" seems about as bad as you could hope for at this point.
Additionally, he mentioned that the company had a reputation for shoddy quality. In plain language, that means the amount of radiation you get blasted with is poorly controlled -- maybe it's the intended amount, or maybe it's much more.
I have objected to those machines on privacy grounds since they were introduced. They are ineffective and routinely fail to detect threats (see link) -- in fact, they fail to detect things the metal detector would! I certainly won't be going through these machines any time soon.
Stand up for your rights, your health, and your safety. Opt-out.