I moved to Japan about 10 months ago and I needed a textbook to learn the language. I settled on the second edition of Genki and I couldn't be happier with the decision. Working your way through the textbook (and workbook) require a lot of time and dedication. That's true of any textbook, but with Genki it's even more so because the authors don't hold your hand and the book is written with the assumption that you are serious about learning Japanese. Be warned, this is not a learn-Japanese-in-a-month style book, nor an EZ-Japanese style book. No shortcuts are taken here. If you are OK with that fact, then I can't recommend this book enough.
The book is divided into 2 sections: "conversation/grammar" and "reading/writing". I'll start with an overview of the conversation/grammar section, which comprises the bulk of the book.
The conversation section has 12 lessons. Each starts with a short dialog or two, then a list of about 50 vocabulary words, which are broken down by word type (e.g. noun, irregular verb, etc). After that comes the grammar section which introduces new conjugations, expressions and other grammar structures. Each grammar point is explained thoroughly, yet concisely, always with examples of typical usage, and oftentimes with more examples to elucidate corner cases or common mistakes.
At the end of the grammar section for the lesson, there is usually (always?) "culture notes" and "useful expressions" sections. There is much to be gleaned from both, though the "useful expressions" sections are usually outstanding and provide supplementary words and sentences on a variety of common topics, such as counting, classroom vocabulary, train station expressions and doctor office visits.
Finally, each lesson ends with about 10 pages of exercises. Some can be done alone, some are meant to be pair work, and a few are designed to be done in groups. The exercises are well-designed and easy to understand.
The reading/writing section of the book, with 12 lessons which correspond to those in the conversation/grammar section, teach you to write about 10 kanji per lesson. Then there are a few pages of practice, in which you will write the kanji you learned alone and in compounds, read various letters or stories and answer questions.
For each chapter, I pretty much went through it the same way. 1) Memorize vocabulary (usually took a week per lesson). 2) Memorize new grammar structures (usually took 1-2 weeks per lesson). 3) Do the workbook and writing section exercises (took about a week per lesson). 4) Meet with my Japanese teacher and do the conversation exercises (took 2-4 hours of class time per lesson).
Thanks to this book, navigating everyday situations in Japan is quite doable. That may not sound like much, but I think it's no small feat for about 6 months of study. I feel quite comfortable in stores, restaurants, train stations, post offices, and the like. I can make plans to meet with my friends and explain if there's a problem. I can talk, to a limited extent, about the weather, my childhood, my job, my future plans, my schedule for tomorrow, and other common topics. I can tell a girl I think she's the most beautiful girl in the world. My Japanese friends tell me that sometimes my grammar is not completely natural sounding, but it's almost always comprehensible.
Is Genki perfect? No. It teaches that パンツ/pantsu means "pants", although after some giggling, my Japanese teacher insisted the word usually means "panties". In 10 months, nobody has ever said "さようなら/sayounara" to me, even though that is the word taught to mean "goodbye" in lesson 1. After finishing this book, I know the words for "astronaut" (宇宙飛行士/uchuuhikoushi) and "anthropology" (人類学/jinruigaku) but not "we" or "they" (OK, I know those words, but I didn't learn them from Genki). But I am picking nits here. It is without a doubt one of the best textbooks I've ever had the pleasure of reading. Anyone serious about studying Japanese will get a lot out of this book, provided he or she is willing to put in the necessary work!
NB: I highly recommend using this book in conjunction with the workbook!