I won this book in a giveaway a while back, and am posting this review.
"Why Love Succeeds..." tries to help the reader understand some of the complexities of the human personality and spirit, particularly when you add the dynamic of another person. Friction and disagreement is sure to come up, and this book looks to aid the person or couple reading it by adding insight into different personality types.
The first half of the book helps establish the personality types of each partner (or even the lone reader), and the second half of the book is short summaries of "If you are personality type X, and your partner is personality type Y.", going through the different combinations. This actually makes a short book even shorter, since the average reader will probably not look much beyond the personalities that are particularly important in their situation.
Generally the book can provide some insight, but it probably shouldn't be used as a diagnostic tool if your relationship is actually in trouble. The quizzes in the first half of the book have a "Cosmo" kind of air, but can be used to get a general lead on a personality type (or at least what you're feeling at the particular point when you take the test.)
The book is probably worthwhile for young couples just establishing that they want a life together, so they can see future bumps in the road.