I didn't find out with DD and one of the most compelling reasons I'd heard was that it can give you extra strength in labour to get you over the edge if you don't know the sex because you'll want to find out.

Must say, that didn't happen for me!

I think actually I would have been more excited (that's not the right word) in labour if I HAD known the sex because he/she would have become a real person in my mind much earlier.

So next time I may do it, dunno.