YES, this book was phenomenal, there is no other word, this book blew my mind! And I have to say it: I liked it even more than I liked Illuminae. 
 Gemina is told with the exact same format than Illuminae, with couple differences: gorgeous journal with drawings made by the very talented Marie Lu, and there is more "surveillance footage summaries" which made the story "more fluid" (and I loved it) 
 I really like the new characters. Hanna is such a strong and kickass character, she is honestly so cool, and I wish I was her (I mean without all the crap she has to go through), and I also really really Nik, he is funny and he is also really really cute (okay, he might be a bit obsesssed with Hanna, but honestly it's not too bad) 
 The new threat was terrifying, and I'm glad we do not have a lot of alien descriptions/scenes because I would have had to sleep with the lights on (what we had was terrying enough okay, it was enough for me to not sleep) 
 And that damn plot twist was insane. The end of this book is honestly one of the best I have ever read, it's mind blowing and really really cool. Well done Amie and Jay, that was amazing! 
 So, if you read Illuminae and you loved it, read Gemina right now. And if you haven't read Illuminae yet, do it and then read Gemina, because trust me it's worth it!