Women:  The woman's opinion on this matter is by far more important than the man's, since it is she that is ultimately the one who has to decide if she wants to swallow or not.   There are many women who truly enjoy swallowing a guy's load.  It often times makes them even hotter and more turned on.  

On the other hand, there are many women who just can't stand the thought of it.  It actually makes them gag, and that, guys, will definitely kill the mood quicker than you can imagine.  Besides, many women would rather see the fruit of their efforts than taste them.  

Men:  All men have some opinion on this matter, and in my limited research, I've found that the majority of men prefer to be swallowed.  However, see above concerning the woman's opinion on this.  If your partner likes to swallow and that's what you prefer, then, lucky you, huh?  However, if you decide at the moment of truth that she should swallow, and she'd rather not, you may find yourself looking for a new partner in the very near future.

All of this is why it's important to communicate about these sorts of things before the fact.  If two people are willing to perform oral sex on each other, why shouldn't they be able to talk about it beforehand?  It certainly does make for a better time, and saves some possible uncomfortable situations later.