I think that Ophelia is one of the most interesting characters in the play. We often focus on Hamet and forget about what she is going through. She essentially has had enough of her crazy family. Who can blame her though?
Consider what Ophelia has experienced: what she has lost, what she has been compelled to do, what she is grieving and mourning. Consider too the outcome of this madness that may seem hard to define as real or feigned. When the two branches of reflection are combined, you have no choice but to understand that Ophelia has indeed and in fact descended to madness and to the snuffing out of her own life.
Indeed, Ophelia becomes insane. When she is in love with Hamlet, she is warned against him intentions by her brother and warned by her father that Hamlet is unstable. Added to these crossed messages, Hamlet himself sends conflicting ones, insulting her one moment and then declaring his love another. Her father's death at the hands of Hamlet send her poor psyche into an abyss of confusing emotions.
I agree that Ophelia really is mad, not pretending to be so. Her madness contributes to the tragic atmosphere of the play; if her madness were merely a pretense, the play would be considerably less tragic than it is. Her sincere grief over the death of her father is something worth emphasizing, since it may cast Hamlet in a far less favorable light than he is usually accorded.
I completely agree with post #2 and would have you ask yourself, "what would be the dramatic purpose of her pretending to be mad?" She has nothing to gain from it, and it would add nothing to the intention of the play. Ophelia is legitimately crazy which actually serves as an interesting contrast to the feigned craziness of Hamlet that we have seen throughout the play.
When one considers the factors that could have led to Ophelia's madness, I think it is quite clear that her madness is indeed real and not the least bit feigned. Her father first orders her to stop seeing Hamlet because he doesn't trust Hamlet's intentions and she cannot simply dally with a Prince unless he is going to make it legitimate.
Then, once she breaks it off, Hamlet tells her all kinds of crazy stuff about how he loved her but he didn't and urges her to get herself to a nunnery since all this love stuff is madness and all men are horribly wicked and evil creatures.
As if that weren't enough, this guy that she loved then acts like a maniac during the mousetrap play, kills her father, and then disappears to be sent off to England. The combination of all of this is enough to give anyone a nervous breakdown and when it is combined as well with her apparent suicide, it becomes pretty clear that her madness is all too real.