I have never faked it. I think it is an awful thing to do. For starters, it's dishonest, and it just doesn't seem like something to lie about in my opinion. Women often don't have orgasms every single time. That's normal, so I see no reason to lie and pretend I did. If I don't have one or can just tell that I won't have one that night, I'll just admit it. I would hate to be in a relationship where I felt the need to lie about things that insignificant.
I also think it is a bad idea because it can give the man the impression that he is doing something right when he isn't. I have heard of far too many women faking it only to regret it later because their partner kept doing whatever it was that he was doing that wasn't actually working for her.
I think it is important to be honest with your partner, whether it is a committed relationship, a one night stand, or anything in between. I cannot imagine having sex with someone that I could not even be honest about little things like that with.