I can't see how finding out ruins the surprise. Its still a surprise - just a little bit earlier. I think you have so many new things to find out when they are born that it is nice to get one of the surprises earlier on.
We found out the sex of DS at 19 weeks. We also named him at this time. It really helped me bond with him before he was born and I felt like he was just as perfect as perfect when he was finally in my arms and I had spent all of that time with him, already knowing who he was.
Ultimately it is a personal decision but I never regretted knowing the sex.