This book will be very helpful for many, since much of it focuses more on spirituality and 'discipleship in the grind' (prayer, forgiveness, simplicity) than on theological issues (i.e., the "Calvinism" vs. "Arminianism" debate). There are, to be sure, theological assumptions made by the authors, but they tend to take a backseat, because the aim of the book is to facilitate discussion between you and your dating partner to provide opportunity for understanding and growth. I am a high-church Anglican, and my girlfriend is a Baptist - you can't be much farther apart on the spectrum! But this book provides fairly common ground for a couple to engage each other, despite potential differences.
The book dives head-first into some issues (within the parameters of its scope), while it merely visits the outskirts of others, such as grace, which I think could have been given a more full-orbed treatment. For instance, the original Greek word for Grace, 'charis', meant, among other things, "the desirability of the beautiful". The implications of this are delightfully helpful for our discipleship and our understanding of God, but are neither mentioned nor explored. There are a few other areas where I could raise this same complaint, but I might be asking the authors to chart out more spiritual territory for the reader than they felt needed to be covered.
I would tend to agree strongly with the reviewer below that this book can be somewhat simplistic at times. The simplicity of the book is, conversely, one of it's advantages, because if you haven't had the advantage of picking up technical terms or language specific to the history of the Christian Faith you'll be more comfortable here. In addition to this, short attention spans and busy schedules are accomodated by the length of the devotionals. Just a two to three page reading each day, with five to ten questions at the end of the week fostering discussion about the readings and relating them to your life. The questions aren't aimed at having the couple regurgitate doctrines, but tease out the inner and outer spiritual life of each person in the couple as an organic thing.
Depending on your circumstances, you might want to give this a spin, even if you're not a young Christian. The first and second chapters I didn't get anything out of, so a moderately well-read individual may need to wait a bit to come across a practical, simple insight which they hadn't considered. My girlfriend and I have found the format of this book helpful (particularly the questions for discussion at the end of each week), in particular because she's not used to either maintaining a steady devotional life (especially not the kind of devotional habits we Anglican Christians have, such as common prayer and contemplation) or being engaged with theological themes. I would expect others to find this book useful, as well, but couples who are more widely read, who openly communicate with each other about their theological/spiritual reading, and who have a common devotional life may want to look elsewhere.