We (the GeoWriting class at OU) recommend this book to beginning scientific writers. Alley's writing style is straightforward, making the book easy to understand. The chapter titles are precisely labeled, allowing one to quickly locate information on specific stylistic problems. Furthermore, Alley writes the way he tells us to write; far from being hypocritical, he practices what he preaches.
Specifically, Alley makes it clear to younger scientists that it is okay to write short sentences. His use of specific examples helped us be more fluid and concise in our writing. He also gave specific advice on how and when to include details, which was an area we had trouble with.
Aside from improving our writing, this book was also useful in understanding how read and critique other papers by providing examples of common mistakes. Naturally, for our own writing, Alley also gives ideas on how to fix those mistakes. Compared to textbooks from other writing courses we've taken, this book includes useful information, rather than merely giving writing examples (e.g., short stories).
On the downside, Alley's examples often focused too much on engineering applications; he should have included more examples from other sciences. At times the examples are also too long and it seems like Alley goes off on a tangent. Finally, while figures used are relevant to what is discussed, the lack of color makes them boring.
Overall, this book is more readable, but less dense than the average textbook. It is worth the money for a course text. We certainly got $12 worth of information out of it.