I thought the more taboo nature of the relationship was handled very well. All three parties involved were considered by each other (to some extent) and there was respect. What I didn't get enough of is "Why are these guys so in love with Madison?" It was hinted at but only briefly and vaguely. There is definitely something about her. I couldn't put my finger on it, but I liked her without having any clear, definitive reason why. But I don't have to have a reason because I'm not claiming to love her deeply. I just felt like it would have been nice to know why these men were so connected to Madison, why they (well, one of them) was willing to do anything and everything for her. I wanted to know why, besides her insatiable sex drive, were they so stuck on her.
I also could not stand the consistent use of "'til" instead of "until." It is such a little detail but drove me up a wall at times. It was clearly an author preference and not a character quirk. Picky, I know...
I really liked the twist thrown in. It reminded me of Gone Girl in the sense that I wanted to go back and re-read certain parts to see if I could have/should have seen it coming. It wasn't anything earthshaking or incredibly mind-blowing, but I really liked the shift.
The epilogue...I could have done without it. It changed the way I viewed Madison, and I don't like that. Again, it was not anything catastrophic, but I don't feel like it fit the Madison we grew to know.
I also don't feel it was necessary. At all. I'm just going to have to pretend I didn't read it and move on.