Here's my take, on that question...
First of all, AJ's character (mostly known as Jack, until near the end) is a complex character, that you really feel for...it's not some flat, 2D character that you just read off a page...he really comes alive and you can't help but feel his emotions. You're instantly addicted to Jack!
The story itself is an awesome concept, and she was really able to pull it off and make it work. It keeps you on your toes and makes you want to look over your shoulder, just in case. There are so many unexpected twists and turns, that you're constantly wondering what the hell could possibly come next. Trust me...you'll never be bored!
The emotional rollar coaster, that she puts Jack on, is another good reason to read the story. She takes him from a cold hearted assassin who doesn't love or care about anyone...who doesn't feel any emotions whatsoever...a nd over time she has Crystal's character slowly awaken these emotions, within him. I don't think Erika could have better portrayed the overwhelming frustrations that he struggled with. His emotions come alive and your heart goes out to him....you want to take this assassin into your arms and tell him everything's ok. The emotions that Erika weaved into the story are so realistic, that you sometimes forget it's just a story.
The most important thing is that people don't let him being an assassin detour them from giving it a chance...sure there is fighting and some killing...but that's not what the whole story is about. It's about a man who has litterally had NOTHING, learning to love, through the most unusual circumstances. He fights it and tries to deny it, but it truely changes him...in ways he can't even understand.
If you're willing to take a chance and throw yourself into Jack's world, you'll be surprised at how much it really sucks you in and how much you end up feeling for Jack.