Personally, I don't think they are worth it unless you live near a nuclear plant. The trick is, you have to get the KI into your thyroid before the radioactive isotope of iodine is ingested- basically, if the thyroid is "full", it has no were to go. Prophylaxis would be the most effective way to go, but your body wouldn't thank you for it.
Keeping that in mind, if I was to live down by Vt Yankee again, I'd have some. It won't block the first exposure, but it will limit the total amount you ingest. Or if I lived down wind of a significant target- I'm not worried about an all out nuclear exchange, but there are odd people in the universe who might some day get lucky.
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-IronRaven
When a man dare not speak without malice for fear of giving insult, that is when truth starts to die. Truth is the truest freedom.