Dave,
Many investment and insurance companies now won't do business if that box is ticked. I know a few people who have renounced it. One was expecting a sombre ceremony, some kind of event where he gets called into a side room for a deal to struck at the final moment. Alas, no. He queued for an hour, went to a table and paid his money. He was then escorted directly out of the Embassy and left without a word at the door in what he described as almost pique, as he was "no longer a citizen".
Why expat Americans are giving up their passports - BBC News