I enjoyed this book very much and understand why so many readers love it. Personally, I have a few reservations and so would rate it a 4 out of 5. 
 What I liked: 
 - World building. The world Sarah J. Maas has created is intricate and consistent in its wonderful details. When a book starts with a map, you're usually in safe hands. 
 - Plot. Overall the plot is intriguing and well-paced. If I can find anything to criticize it's the "exposition dump" once the curse has been lifted and everything can finally be explained to Feyre. However, I can understand why the author did this. It would have been very difficult to reveal this information earlier without undermining a crucial plot point. 
 - Characters and sexual politics. I give ACOTAR big points for giving us a heroine who is comfortable with sex and her sexuality. Also, she is treated as less capable because she is a woman. Any discrimination is along species lines. Anyone looking down their nose at Feyre and sneering at her weakness does so because she's human. 
 - Dialogue. Even though we're in a world of faeries and magic and goodness knows what, the dialogue never sounds forced or clunky. 
 So... why couldn't I give it a 5/5? 
 In recent years I've seen so many good movies which could have better (in my humble opinion!) by cutting 20-25 minutes. Similarly, I feel this book would have been better is 50-60 pages had been cut somewhere. Perhaps in the middle section, or in the final quarter of the book in the court Under the Mountain. 
 [SPOILERS AHEAD!] Coming back to that final quarter. I know it's supposed to be incredibly dark and nasty (we're under the mountain after all), but I would have appreciated a bit of relief. We are talking Game of Thrones levels of gory violence and sadistic cruelty sustained over a quarter of the book (I checked, it really is a quarter of the book). I can understand why the author wanted to make Feyre go through three tasks (power of three in fairy tales and myths and all that), but I could have done with just two! 
 And finally the core problem. To really fall in love with this book, I think you have to fall in love with Feyre's love interest: Tamlin. And I didn't. Perhaps this is because I'm a 33-year-old and not the book's core audience, but I found him a bit dull and overly preoccupied with how damaged he is. I'd rather have his witty friend Lucien or his dastardly rival Rhysand (devil with a heart of gold - who can resist?). So when Feyre is going through some incredibly nasty stuff in the last quarter of the book to save Tamlin (and basically the whole world, but mainly Tamlin), I found it hard to get behind her. 
 That said, it's a great book and if there's more of Rhysand and Lucien in the sequel, I'll be first in line to read it! :-)