The beauty of multiply is not in its unique approach to discipleship, its discovery of a hidden facet of training up strong followers of Jesus, or even in its back to basics approach. The beauty of the book is found in how easy it is to repeat with others.
One of the other reviewers, who was basically positive, mentioned that this was no different from Sunday school curriculum. In a lot of ways, he is correct. The difference is that this isn't dependent on a lay-leader or "Christian professional" to use. If used properly, it can be a powerful tool in the hands of your everyday church attender.
How we are trained is often the way that we will train others. If you were trained in a Sunday school, than you will, most likely, bring your neighbor to church and let someone else train him up or tell him about Jesus. With multiply, you could (and should), have your disciplee begin discipling someone on the first part of the book before you are even done going through it with them. They should, in turn, encourage the person that they are discipling to do the same. This has the potential to create a culture of discipleship whereby it spreads as a way of life instead of a forced program. That, I think, is the true power of the book. You don't need to reinvent the wheel and write your own discipleship study or train 10 guys on how to do discipleship. The book provides all the tools necessary (especially when coupled with the website) to equip someone to disciple another person. Even if the person you are discipling isn't done with the book, they are equipped to wrestle through the first few sections. By the time that you have finished going through it with them, you are in a position to monitor and assist them as they go through it with someone else. You learn by teaching/doing, and they do the same. Doing this doesn't pigeonhole discipleship as a job for the professionals, the pastors, or the teachers - it puts discipleship in the hands of every person.
The book covers the basics of what people need to do know about the nature of discipleship (what it means to follow Jesus), the Bible, and the church. Right off the bat, I have to give kudos to Chan for including a section on the church. One of the downfalls of so many discipleship materials produced over the last few decades is a glaring omission of anything related to the church.
All the material is available for free on their website, multiplymovement. This is comforting to me because it gives proof to the point that they aren't trying to make money by getting as many books purchased as possible (with a discipleship method reliant on their book). You can download the pdf chapters and print them out at your leisure, watch leadership videos, and even stream Chan reading his book to you - all for free. Incredible.
Too Long Didn't Read Version: Multiply is great because it puts discipleship on the bottom shelf, not by diminishing the depth of content, but by making it accessible to everyone - not just pastors/leaders. I truly believe that this could be a discipleship tool that anyone could use to disciple another person with little preparation beforehand.