From a guy's perspective, I totally agree with Donna... go for it! I love seeing all the lovely ladies dressed to the "nines." The formal nights are really neat and you will not be over-dressed in a long gown. The passengers on Princess seem to take the formal nights a little more seriously than some other lines.
Be sure the "ole man" is decked out in a tux to go along with your formal gown. If he does own one, no problem, you can rent them through Princess and they will deliver it to your room and pick it up when you leave. It cost about 90 bucks for all week with two formal shirts and all the accessories.
As Donna said... be sure to dress early and get to the portrait locations early. The posing is free. They will take two or three shots. After they print the portraits, you decide if you want to buy any of them. My wife and I have had the best photos made of us on board the cruises.
Have fun and post a picture of you and your gown.
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