At the very first place, let me just say oral sex is illegal in some jurisdiction. Some notable development would Indian Court had just recently declared its unconstitutional to ban gay sex (read SODOMY), while it is legal for consensual heterosexual oral sex in Singapore (previously illegal, though everyone is doing it), while in Malaysia, it is still illegal, and yes, i think everyone is doing it too.
However, oral sex is only illegal when performed on man, it is perfectly legal to perform it on a woman. Talk about gender equality.
Now, what so good about oral sex?
For a man, the feeling of his rod being sucked at like the most delicious candy is surely great. In anyway, there is a lot more thing you can do with your mouth than with your pussy. One can press all the sensitive point with her mouth, something a pussy can never do. And the thought of a girl willing to go down for you.
For a woman, i believe, the reason is the same, though finger might do a better job.
I think the best part of oral sex is the psychological aspect of it. The meaning of someone willing to go down to a place one would normally would not go with his/her mouth and tongue.
Afterall, sex is not so much on physical aspect but more on mental gratification. What is perceived sometimes works better than what is in reality, or at least in women's mind, it does.