Loved it! The magic system was well thought out and fairly original - I loved the math/geometry aspect of it. And Sanderson managed to make chalk outlines creepy. I thought the world building was well done, and particularly enjoyed the little hints and bits of steampunk. I was super intrigued by the alternate version of the Americas, both the actual geography and the impact it had on politics. I would definitely be interested in finding out more about that. 
 I also really enjoyed the characters. I liked Joel's persistence when it came to Rithmatics. Even though he knew he wasn't a Rithmatist, he still wanted to study it, and wasn't willing to give up that goal. I thought the way his friendship with Melody blossomed was really well done, and natural. I like that (view spoiler)[when Joel went through his inception again, he still wasn't a Rithmatist. I think Sanderson could have gone that route, but it would have been too easy. Obviously, there is something else going on with him in regards to that, though, and I'm certainly eager to find out (hide spoiler)]. 
 The mystery aspect was decent, though it definitely felt secondary to the character, the world, and the magic system. And the illustrations of different Rithmatist defenses, along with their explanation - absolutely loved them. I liked the extra insight they provided into the magic system. 
 I'm definitely looking forward to reading more of the series. 
 Note: So I won this book through Goodreads' First Reads giveaway program. Yay! 
 Also, please excuse the disjointed, rambley review. It's 2:30 am and words are hard :D