Just finished the book and I am a little disappointed.
I have read Brian Greene's previous books and although his previous books are the best I have read in theoretical physics especially for the layman with no PHD who is just interested in the subject, this one compared with them falls short. I have always enjoyed the ideas of parallel universes and sci-fi programmes that have incorporated this topic from sliders to episodes of star trek, and this book as you can refer from the title does cover these parallel universes and the variations of such universes.
The first 1/3 of the book I found really interesting and had a good flow to the different theoretical parallel universes that have been conceived, but maybe the book should have ended there, it would have been short and sweet. In the end the book felt more like it dragged and began to repeat itself quite often. I think it strayed from the path it started out on, the book should be condensed. I prefer to read quality rather than quantity!