This was a tricky one. I'm usually a fast quick reader, but this took me a couple weeks to get through, and it was such a slog sometimes. So what happened is... this book is paced terribly. There is a review further down by @anotherreadingblonde that really sums it up nicely: the potential is great, the pacing is terrible.

I am a huge ACOTAR fan (and I guess ToG is ok), though not a huge fan of Aelin (at all) and Feyre (mostly). The way this story (and universe) are built is *very* similar to SJM's books. You got your mysterious and tortured assassin FMC who turns out to be not only fae, but a (previously lost) fay queen. You got your various courts with their inner circles. You got your Rhys (Sorin), Cassian (Cyrus) and Azriel (Rayner); your Mor (Eliza), your Dorian (Callan) etc. You got a mysterious people that may be invading by taking positions of power - the Maaren Lords (valg), and so on. You even got the explanation of different "planes" and beings that can walk through these universes. 
So if you loved ToG and/or ACOTAR and don't mind reading more of that, this is a great book for you!

That said, the pacing here is horrible at times. I almost DNF'd this multiple times, and at the end of book 2 I want to know what happens next, but I'm also really hesitant to jump into another one bc it was so hard to keep reading at times!
Scarlett is depressed for most of the time, which, fair, but also gets a bit old. Compared to Feyre's healing, or Nesta's, this is so hard to get through... there is my nemesis, the miscommunication trope. It's so bad, and so frequent. Yes, all are tortured souls that had to fend for themselves, but at some point in your hundreds of years of life you must have learned to communicate better than this?!
The handling of the age gap is fine for the most part, though <spoiler> there is one point where the "you really are ancient haha" banter turns pretty cringe. Sorin says something like that he could have saved her from all the trauma she went through, being raised an assassin etc, if he had only known Elinè had had a child, and I was honestly not sure if this 300+ years old dude would have then, what, raised his twin flame from early childhood (around 3 years old)? It's called grooming, my friend. Plus, you were soulmates with her mother on top of that (platonic in this universe, but still a twining of souls)! Like... ew. No. </spoiler>
And the final thing I really hate is that Scarlett acts like Aelin, and I hated how Aelin acted. The deus ex machina moments ("while you were trying to help I acquired the necessary knowledge but didnt tell anyone, but LOOK here is the solution", "there is this notoriously unhelpful character that has been awful for hundreds of years, but we hit it off immediately and they will help us of course", etc). I thought these surprises in ToG were lazy writing and bad planning, ie. you need to get your story to some point so you just magically make that happen), and I didn't like them here. 

So where does that leave us. I think this world has huge potential, I just wish it was paced better. I get that some of the endless explanations are necessary to round off a character or make sure we understand the gravity of some future plot point. But I've rarely found a book easier to put down than this one. I even started reading two (!) other stories at the same time, which is unusual for me. 

I don't regret reading it, but I think I might need a break before delving into the next book. Should you read it? If you're someone who likes these expansive worlds, you will definitely enjoy this one, and it being a finished series is a huge plus here as well! If you need plots to move and don't have a lot of patience for the in-between details, you may wanna pass.