When I was extremely happy to find this book at a used book stand for three dollars, my fiance asked me what it was about. I told him "I'm not sure exactly, but I know it's about two people growing up during World War II, and every one has said how good it is."
A week later, when I had finished it and was gushing about how wonderful it was, he asked me again. "It's about a genius orphan boy in Nazi Germany, and a blind girl growing up in Nazi-Occupied France. And radios. And triangles. And a cursed diamond. But mostly it's about how our lives intersect in a million little ways with the lives of others." He replied that it sounds complicated. I said, "Not so much."
The truth is that it's not really complicated at all. Despite flashing back and forth both in time and between characters, All the Light We Cannot See is very easy to follow. Characters' names are distinct, and the way that the characters look at and interact with the world is different enough that it's not at all difficult to remember if you're in France with Marie-Laure and her father, or in Germany with Werner. The characters and their environments also change enough that it isn't difficult to remember 'when' you are, either.
I have read reviews that complain that the ending is confusing, and you never find out where the diamond actually is, but I disagree. The ending is pretty straightforward, and there's even a little section at the end from the "POV" of the diamond, to let you know exactly where it is and what happened to it.
This book is beautifully written, and it deals with life in WWII Europe in a way that feels authentic and calls for minimal suspension of disbelief. The characters' actions, struggles, and fates all seem very real, even as you ponder whether or not the "curse" ever actually existed.
Basically, read the book.