Just my thoughts
Humour is a subjective matter and what some find hilarious, others find extremely boring.
That being said, I'm a 18 year old male, cinema aficionado and rom-com hunter. And I honestly think that after all the romantic comedies I've seen over the years, this one definitely has to be one of the very best.
I didn't like it because someone told me to do so, before watching it I didn't know anything about the hate or the love for the movie. I didn't even know the plot. And I couldn't help feeling helplessly captivated by Hepburn's character.
I've read many things here: that Susan is annoying, that she is the opposite to what any man would want and that her character has no foundations on reality...
That she's annoying I can't deny nor confirm, because well..., again, it's subjective. I know what being annoyed to death by a character is, specially the voice: Cate Blanchett's Kate in The Aviator totally ruined the movie for me the first time I watched it, and then the second time I watched the film, I found her lovable (and it was that what made me watch The Philadelphia Story and now Bringing Up Baby), there also was Emma Stone, Margot Robbie and Heath Ledger's Joker (three actors who now I love); and currently I'm in the process of hating John Malkovich and Jennifer Jason Leigh (all because their voices). Thing is, it's subjective and we have no control over it (at least I don't), so I don't think it can be used for or against the movie (though I must say I loved her voice).
But the two other points I think I can refute: I have found people that truly are crazy like that, although not so much as Susan: it's a comedy, therefore her character is exaggerated and caricatured for humorous purposes. But, still I could see part of them in her; so to me she didn't seem so fake or made up, she was merely a comical hyperbole.
About she being the opposite to what any man would want, what's up with that? Like there is some sort of predetermined personalities that men would want and some that would be destined to die alone... that isn't the truth. Her personality, her vivacity, her energy, her child like excitement, innocence and lack of care... It's perfectly alright if you didn't like her but it's unfair to say others can't. I'm not saying these are all good qualities but they do have something to them that attracts me. Very rarely do I see people like that and whenever I do, like this movie, they usually fall into that category of either you love them or you hate them.
Frankly, I can't help but feel fascinated by Susan. Much like Zooey Deschannel's character in New Girl (I know it's a meh quality tv show, relax, I'm just using her as an example), Audrey Hepburn on Breakfast at Tiffany's, Grace Kelly on Rear Window and our beloved hobbits, she is so high on life, and that (at least for me) is quite contagious and makes a wonderful movie.