I got this book, hoping that it might be a good way to quickly review some of the basics of Greek grammar. Not so. I think that this overview of New Testament Koine Greek is too rudimentary to really be helpful to anyone, even/especially people who primarily study the New Testament. I'm a Greek student, although I primarily study Attic Greek, and I couldn't help but feel that this book glossed over some of the nitty-gritty (but still important) grammatical concepts. I was really bothered by the participles that were constantly left dangling. During the chapter on the Genitive Absolute, Countryman says that equivalent phrases in English don't sound very good, but I posit that they might have sounded better if he had construed the circumstantial participles.
Here's the deal: in Greek, the grammar matters. In order to have a good understanding of a Greek text, it's important to be able to identify the grammatical form and function of every single word in the sentence, and it's not always clear by looking what each word is doing. The oblique cases (Genitive, Dative, and Accusative) have tons and tons of usages, most of which aren't even touched on in this book. Some of those usages require prepositions, which is also not touched on. This sort of thing matters when you're trying to understand the text.
I don't know what Countryman's educational background is, but it read to me that he didn't know enough of the grammar to be offering tips. He wanted to simplify something that is by nature incredibly complex, and he really missed the mark. If you're interested in studying Greek, I'd recommend starting with Greek: An Intensive Course by Hansen & Quinn. It's a much larger book, and it's certainly not as friendly, but it will help you learn the language. This book will not.