My main issue was with Caddy herself. I really didn't like her at all, finding her selfish and whiny. I understand people often feel discontent with their situation and long for excitement, but Caddy came across as jealous and petty. She also doesn't seem to understand the different situation her friend Rosie is in to her. Her main complaint was the lack of anything bad happening to her!
I understand the potential dilemma of your best friend making a new friend, so Caddy's uncertainty wasn't too surprising. She feels as though this beautiful, popular new girl is going to take everything away from her. The paranoia and squabbling between the three girls is pretty accurate and I was happy to see Caddy realising she could have another friend in Suzanne, rather than loosing Rosie.
Suzanne isn't easy to like either but her issues are more deep rooted - she has actual problems rather than simply wishing to be more interesting. I found myself waiting for something terrible to happen (don't worry no spoilers here). My favourite part was how the book ends, it seems fitting within the scope of the story. I liked the message told in Beautiful Broken Things - the concept is dark and clever, but for me I just couldn't connect with the characters and it all felt a little awkward.