I love stories like this, bad guys who are really good guys. It's the YA version of Oceans 11 or Leverage. 
 Obviously the cons are completely outrageous but they are really fun. They do so many cool things and what's even better is that they don't use a whole lot of special tools/gadgets to get it done. I wished they would have explained the cons better after they were finished (the ins and outs of how it worked). That's my favorite part of Leverage. 
 I was a little bothered that Kat was only 15. I know that the whole premise is not "real" but it seems a bit realistic that a 15 year old could get around like she does (there's a lot of traveling to different countries). Hale didn't seem 16 either. Honestly in my head I made them all in the 18-20 range because for me that made more sense. 
 There were some romantic instances between Kat and Hale but not a whole lot of romance. They were super cute though. This area of the book was the only one that I felt fit their age range. 
 Kat was a really good main character. She wasn't all "please save me" or "I can't survive without my boyfriend." At times she was the exact opposite, going to far to be independent. She maintained a good position as leader of her group of thieves. 
 I really enjoyed Kat's friends. They were a good group that fit well together and played off each other very well. There was some good interaction and a couple of good one liners. Similar to Oceans 11 and Leverage they each have their special unique talent. Gabrielle was my favorite, she went a little above the call of duty. 
 The bad guy, Arturo, wasn't as bad as he could have been. He wasn't really a major player in the story. 
 The best part was the final heist at the end. It was set up perfectly, but then of course there are problems, some of which are funny, while others are more revealing.