It's still a suprise whether you find out or not - you just find out a little earlier!
For us our reasons for finding out were:
* We were impatiend, so had a 4D gender scan at 16 weeks
* It's made it real, made him into a person, given him a personality a he, a Leni - not an 'it' or 'baby'
* We were able to choose a name, and not call him 'it' or 'baby'
* I believe we have bonded with him incredibly and wouldn't have been able to do so as much if we didn't know 'what' he was
* Choose gender specifif clothes/furniture/pram etc
I will find out the sex next time too, i would hate no to know!