As a person who trains in a lineage descended from Shaku Soen, I can say that this book helped alot in my understanding of my own lineage. In particluar, the expositions on the Dharma-kaya, on death, and on suffering are very well presented. The sections comparing Buddhism to Christianity alone are worth the price of the book.
That said, however, some of the concepts that are presented, in my opinion, are not rendered well in English. Too often the term "monism" is used, which has implications that have been seized upon by people that would rather create a caricature of Buddhism. ("Non-duality"- not two, and not one- is a better rendering). Unfortunately, Suzuki was an early translator, and this is an early translation of Japanese Buddhist ideas.
In addition, we see in this book some of the infusion of Japanese martial spirit that was later to tarnish Japanese Buddhism, and to provide Westerners with an important koan- how to reconcile the Dharma of compassion and respect for all beings with the militant nature espoused here. There is an answer to this koan, to be sure, but you'll only find the question hinted at here.
So, I'd recommend the book heartily, but the reader should also read other works to get a fuller picture- e.g., Brian Victoria's book on Zen at War, as well as Nagarjuna, and other writers.
Don't let this book's title mislead you--this is a wonderful introductory-level book not just to Zen but to Buddhism in general, presented in terms that make it easy for the Western mind to grasp. It does so by using Western religion, philosophy, and psychology as vivid points of reference for comparison and contrast. Given the time frame in which these lectures were given (1905-1906), when Buddhism was new to virtually all of the Western hemisphere, this approach made sense then. But it still works today. The book eschews the nearly impenetrable nature of later books on Zen, instead cutting right to the heart of what it means to be a Buddhist, in practical language. Rather than dive into technical explications of individual teachings, such as the Four Noble Truths and the Eightfold Path, Mr. Shaku prefers to simply explain the spirit behind them, which (as with all basic Buddhist teachings) is to realize enlightenment and help to lead others there. Thus, this book isn't the best for those who want the minutiae--historical backgrounds, lists of doctrines, and so on--but there are plenty of other books out there that do that job superbly.
Although today's committed Buddhist may be uncomfortable with the book's many allusions to "God," it's once again clear that invoking the God-concept was the most pragmatic way for Mr. Shaku to draw parallels for his audience to Buddhist metaphysical ideals that reflect similar notions of the absolute. It's for this reason that this remains an excellent book today for Americans who follow Western monotheistic traditions and need a good reference point from which to place Buddhism in the perspective of their psyches.