I really liked this book.
It's easy to read, and can be read at leisure (I find it hard to concentrate on some text books). Its quite small, making it attainable to read the whole book, and it will improve the way you approach experimental design.
I'm a PhD student, so have designed a few experiments in the past. This book really pin points what you should be thinking about and why in a clear concise way. Its good at emphasising why you should take time to do certain things, and also what you should do if those things just aren't feasible. It also gives examples to illustrate points.
Is it essential? Probably not. I'm sure most people in science are aware of the key concepts, but this book reminds you of them, which is important.
Is it helpful? Definitely. It helps focus all the things you know about, but don't quite have a full grasp on.
Will it help you design better experiments? I'd say yes, and if, for whatever reason, you can't follow its advice, it will prepare you for the types of criticism you will face.
Who is it best suited for? I'd say people doing a life science degree, master's course or beginning a PhD could benefit from this book.