So, I read this. I've heard many different things about this book and I was eager to check it out. Besides, I love seeing LGBT characters in YA, especially when they're the main characters #WeNeedDiverseBooks
Let's get something out of the way: I haven't read Fangirl but that didn't make things confusing. You don't need to read Fangirl first to understand Carry On, in my opinion.
Another thing I should address: this book is very similar to Harry Potter. I think we all know by now that this is basically Harry Potter fanfic. Some people thought the lack of originality concerning the world building was irritating, and I can see why they thought that. But me, I liked seeing the parallels between Harry Potter and Carry On. I liked seeing the similarities and differences. Besides, not everything is the same. Rowell adds her own unique touches to the story and I liked it. The world-building is the major thing I liked.
Now, characters. Simon is likable enough, but pretty boring for the main character. His chapters were only okay, nothing more. Baz was way more interesting. He's sarcastic and snarky but also he has a soft spot for Simon, and that's so cute. Penny is awesome, she's everything I liked about Hermione with none of the qualities I disliked. I hated Agatha. She was a completely useless character, and beyond that she's annoying. She reminds me of Cho Chang. She's only there to create angst and drama, and has little personality beyond that.
I was so looking forward to the romance. That's one of the main reasons I picked this up. I wanted to love it, I did. But...I didn't. It could've been better, imo. I guess the enemies-to-lovers trope isn't really for me?
Oh, I totally called the ending. I kept thinking, there's no way I'm right because I never guess the ending right. Until now! I'm so proud of myself for that accomplishment I'm not even annoyed it was so obvious an oblivious person like me guessed.
Lucy's POV was confusing. You don't really see how her chapters fit into the story until the end, so that was irritating. Besides, was she even a neccessary character? I think the story would've been fine without her. Also, what happened to her? I(show spoiler)
Tl;dr version- In the end, I'm glad I read this, but I probably wouldn't read it again. I liked it, didn't love it. Awesome world-building, some great characters, but the romance wasn't spectacular and there were some not great characters. But I do recommend it.