Two reasons I changed my 'definitely not finding out' stance.
Firstly because DH REALLY wanted to find out. He wanted to find out more than I didn't want to, and I knew one of us finding out and not the other just wouldn't work.
Secondly because I was becoming SO SO convinced I was having a boy. I worried that if I went through the whole pregnancy talking to and bonding with my little boy, and it turned out to be a girl, I would be disappointed.
Now, I'm really glad I did find out, because as I'd feared, she WAS a she!! I was so incredibly surprised when the sonographer told us, I didn't believe him at first. I was a little disappointed to be honest, but probably more so because I knew DH really wanted a boy first. But as the pregnancy progressed I got really excited about having a girl, and now the both of us couldn't imagine (or want) it any other way
We did decide to tell everyone that we hadn't found out though. It was our own little secret (though we were very close to letting slip a couple of times).
Good luck with your decision!!