Green is promoted as the book that is both the beginning and the end of his latest four book series. Supposedly you can read this book without the other books and still understand and enjoy it. Unfortunately, if you're not familiar with the other books, including his previous books, and the characters, you can find yourself rereading passages to find out what you're missing.
Dekker's forte is in writing action scenes. The end of the book pulls you in and definitely keeps you reading as the characters race to meet their destinies. Unfortunately, much of the rest of the book drags. Some of this is due to the confusion of moving not only between worlds, but between the beginning and the end.
To add to the confusion, there are many unusual characters with specific histories. At times, I found it difficult to keep them, and their various roles in the story, straight. By the end, I felt I had figured out the inhabitants of the future world, but the middle was often rough going, particularly if you had to put the book down for any length of time.
However, if you like fantasy with a Christian background, you'll enjoy the book – and probably want to read the other books in Dekker's series.