The premise of I am the Messenger is about this guy named Ed Kennedy who is just a very mediocre guy. He's not very good at anything but not totally awful at everything either (except sex. He mentions that he's terrible at sex)--he's just a regular Joe. He works as a cabdriver in a relatively small town, has three close friends, one of which he is hopelessly in love with, and a very smelly old dog named the Doorman. Early on in the book, he tells the readers that cards play an important role in his life--he would often have his friends over to play cards and he would soon discover how important cards would become.
One day, he discovers a single playing card in his mailbox, the Ace of Diamonds. On it were three streets. Thus begins Ed's story as he goes to these various locations in order to help people. Each case contains a different person and each person has a different problem. As you might imagine, he receives the other three Aces, each slightly different in terms of clues and Ed continues to go around his town and help people.
The plot itself is really not bad at all. I like how it works out and the process is very suspenseful at times. However, the bad part is the author's writing style. Zusak is a good writer and his style worked beautifully for The Book Thief but for some reason, it just didn't click with me in this book. The short brief sentences that made his descriptions so powerful and emphatic just didn't seem to work as well for I am the Messenger. Keep in mind, this is just my opinion--many of my friends say that they loved this book and thought the writing worked fine but I just simply didn't enjoy it as much.
Also, there were several points in the plot in which I thought to myself, "How the hell is that even possible?? Nobody would ever react like that! Come on!". Perhaps the author realized this and only added it to add to the dramatization of the scene but I generally have a problem with unbelievability when it comes to realistic fiction.
That being said, I don't think that this is a bad book and isn't necessarily a waste of time to read. It has an interesting ending and certainly a very engaging plot line. I'd classify it under the category of "good time passer" but not exactly a "must read before you die". It's up to you. Good luck.