Fangirl by Rainbow Rowell is a well-written engaging book. I liked the characters (even the annoying ones – they served a purpose) and the semi-dramas that happened to twin sisters in their first year at college. This is real life stuff: first 'real' kisses by boyfriends, leaving home for first time and the family baggage that comes with you. Add to that a broken home (where the mom left) and a dad living on the edge of crazy. No wonder Cath, the main character, likes to live in a make-believe world created by someone else.
I'm not at all familiar with fanfic but I'm guessing there was a lot of this written after J.K. Rowling finished the Harry Potter series of books. HP was such a phenomenon that no one wanted to see the books end and many, I'm sure, continued the story long after JK put down her pen (or shut off her computer).
RR mentioned having read a lot of fanfic and I guess you would have to if you make it a major theme that runs throughout your main story. Since I'm not a fan of fanfic I would have liked a little less of the fanfic that ran throughout the novel. But the name of the book is Fangirl so Rowell had to include enough to make her character a convincing fan, which she did. Cath's professor accused her of plagiarism – I don't completely agree with her but I likened it to, as a fine artist, someone creating a painting from one of my paintings – doesn't seem right somehow. But I found all of it fun and stimulating enough that I was looking forward to the next chapter in the 430+ pages.
I rate Fangirl 4.5 out of 5 stars.