That's going to hurt a LOT. To the point where she might even scream in pain. PAIN not joy. That's why 2 fingers in a girl is already enough, why would a fist make it better? Even 3 fingers hurt a bit. Don't do that to a girl, you'll just put her in pain.
Some girls like it, and those that like it probably think it's amazing. It seems like a love it or hate it type thing, because it's so extreme. Personally I wouldn't want to do it. I think the majority of girls probably wouldn't.
Ah if a dude ever tried that with me he would find his fist somewhere else where the sun doesn't shine. That would be horrible, I don't even think its physically possible unless the girl was exceptionally loose. It just sounds terrible.
1) just because you see it in p*rn doesn't mean it exists in reality2) no I do not like it3) that would probably hurt think of a babys head as the size of your fist4) it will make her vagina looser and stretch it out, which makes sex unpleasant especially for YOU