I've never been punched in the gut by a fic. I can't really imagine what would have that sort of affect on me, either. I always see these comments by people about stories (fic, or books, or movies) bringing them to tears, or more significantly, staying with them for hours, days, weeks -- but I don't know what that actually feels like.

I know what it feels like to tear up a little when a character dies in a really well-written fic, I know what it is to ache along with a character over relationship angst/unrequited love, but not to have it stay with me.
I don't think I'm ever completely immersed in a work; there's always part of me that's looking at how it's written. I go to plays and half the time I'm thinking about the scenery or staging rather than the actors. With movies it's the music and cinematography.