For me, Breaking Dawn is the greatest departure from reality. It requires a greater "leap" into the world of fantasy than the other books in the series. I felt like even though the previous three books are fiction, they still had elements that grounded them in reality.
However, most of that goes out the proverbial window in Breaking Dawn. There are several elements of the story that require one to turn off the rational part of your brain and simply accept them as fact - such as the fact that Edward, an undead vampire, having the ability to create live offspring - in order for the story to work.
You have to be willing to make that jump with Stephenie if you are going to accept and appreciate Breaking Dawn. I have to admit that the first time I read Breaking Dawn, during the marathon weekend when I read the whole series, the departure from reality didn't bother me so much because I was already so lost in the world of Twilight. However, after subsequent readings, I found myself questioning the story more, and it started to ruin it for me...until I turned off the rational part of my brain.
I think that all book lovers find a book or series (I have several!!) that we come back to again and again because it offers us what all great stories do - an escape...a chance to go places we can only dream of, to see things we can only imagine, and to do things that we may never have the real courage to do. Twilight is one of those series for me...
People always ask me why I read it again and again. It's because I don't have to rationalize it. I have a job that requires me to make rational decisions all day, and I love being able to turn that side off once in awhile. I love it because I love the characters, and the fantasy, and the cliche theme of love conquering all and living happily ever after.
I did see Breaking Dawn - Part I and I wasn't disappointed. There was some artistic licenses taken and some things that I didn't care for, but overall I feel it was fairly true to the book. But fair warning...the movie also requires a "leap."