In order to help you decide whether or not to buy this book, I will try to make clear what the conflicting reviews mean. Actually, they are all right (in my opinion). Which side you take depends on the method of learning you prefer.
If you want a quick, "no brainer crash course" that will get you up and drawing ASAP, then use first "The (New) Drawing on the Right Side of the Brain" by Betty Edwards. The exercises in that book can be completed in less than a week; by then, you should be able to draw impressive lifelike portraits.
Now, let me warn you: Ms. Edwards' book teaches you how to COPY (draw) WHAT YOU SEE just as your eyes see it. You won't learn to draw from nothing; you will need a photograph or model to work from. Simply put: you will learn to copy whatever you look at.
Now, if after that, you find yourself wanting for more-- that is, you want to learn and do more -- then get this book by Nicolaides. This is a serious instruction manual that requires a lot of your time and energy. Using it is just like being in art class. You have to follow 25 schedules amounting to 15 hours of drawing each, and in all you will use more than 60 exercises. Each chapter builds on the previous ones, so it is necessary to do all of them in order, for as long as directed. This will take 6-24 months to "finish", assuming the student draws 1-6 hours a day.
Does that sound too much for you? If so, don't feel bad. This book turned me off, too, when I first opened it. It does take a lot of work; I understand why some people are disappointed by it. But if you keep up with it, you will definitely see the results at around Schedule 13. Several chapters after that, I found myself experimenting with all the drawing exercises I'd learned (Nicolaides, Edwards', Pogany's, etc.) to make the drawings I wanted. I also use computer programs that Nicolaides never even dreamed of.
That -- learning to combine and/or make your own drawings and nost just plain copying -- is what puts Nicoliades' book at a different class from Edwards'. That, and learning to experience the model in the natural, if old-fashioned, way.
So, my advice is go first for Betty Edwards or maybe "Drawing For Teens"(?) recommended below by an earlier review. If you want more than that, then come back here and get "The Natural Way to Draw". It might bore you at first, and that is nothing to be ashamed of. But give it the effort it deserves and you will know why this book has been called "not only the best how-to book on drawing, it is the best how-to book we've seen on any subject."