We are doing the "we know but aren't telling" thing.
For us, it doesn't matter either way (boy or girl). That is, doesn't influence our choices in nursery decoration or such stuff. Given that it didn't matter either way, we figured "why NOT find out" - particularly as that will help us save time in thinking about names.
However, it seems that it does matter to other people. Some people are saying things like "you are just making it hard for us to buy things for you". Well that is precisely why we are not telling them. Because it apparently WOULD influence their choices. I'm a firm believer that a baby's sex should not make the least difference in what he/she wears, plays with, looks at. I wouldn't mind dressing a boy in pink or a girl in blue, for that matter.
I don't like lying, so when people ask if we know the sex, I do say yes.
Somebody on this thread said, why not tell strangers, they are only asking to be polite and it won't make any difference to them, whereas saying "I'm not telling" is rude. Well the problem is, I would find it odd telling strangers but not family. So once we decided to not tell, I'm afraid strangers just have to live with the "rudeness" - I hope they do understand that it is not them we're not telling, but everyone.
As for experiences with this strategy, it has been mostly good. People tend to respect our choice (and say so!) and if they don't, well then that is their problem really, no? There has been one exception - DP's sister. She told us in quite clear words that she didn't approve, that she thought we were being silly and spiteful, wanted to make things hard for everybody else, and that she wasn't about to excuse this, as she sees it, childish and egoistic behaviour. I guess as she is pregnant as well, she can be excused though .
Maybe, just perhaps, we will start telling once we have settled on a name. We can't keep it a secret forever after all... and DP's father is itching to make stuff with the name on it (he's a phenomenal wood worker).
But on the other hand, I've heard so many stories where the predicted sex turned out to be wrong. Especially with the latest scan being at 20weeks.
I really liked how Roger Federer handled it... rumours had been his wife was expecting a boy. Turned out it was twin girls.