First and foremost, understand what this book is. It would most properly be termed a graded grammar. It is not a book to teach you to memorize how to ask the location of nearest bathroom. The focus is on the written modern language, with some discussion of classical Persian as well.
Now, because it *is* a grammar, it uses actual grammatic terminology. Some reviewers are apparently shocked by this. So, if you're not familiar with that terminology, you may actually have to use an English dictionary to look up some words. It's pretty difficult to learn a language on your own without understanding some grammar. There are very expensive and time-consuming courses in some languages that will drill you through all the various grammatical forms without explaining what they are, or using any hard English words (but nothing like this is available in Persian, anyway.) So, get over it, and use a dictionary.
I'm using this book to learn how to read Persian. It's pretty good for that, and quite thorough. It would be useful as an adjunct resource if you want to learn to speak Persian, but not as your primary resource. It doesn't have the appropriate sort of drills and tapes for learning to speak. The drills are of the more traditional two-way translation variety.
The major weakness of the book, as others have pointed out, is the lack of keys to exercises. For me, it's not a big problem, because if you're learning to read a language, you can usually tell when you've figured out the right translation, because things will just "click". If you wanted to learn to write to your Persian friend in Persian, this would be a major problem, as you really need a key to the exercises, because you'll make little grammatical errors that you won't catch without a key.
A minor weakness is the presentation of the alphabet. Everything you need to know about the Persian script is presented in the introduction to this book. However, it's presented in a very concise format, so what you'll have to do is use this information to make up your own drills with flash cards, etc., so you have a good handle on the alphabet before you start. That's what I did, and it worked fine.
You may also want to either get the tapes associated with the book, or get another course where the focus is on speaking. I say this only because I've had real trouble in the past learning to read languages where I didn't have a firm grasp on what the language sounded like. For some reason, I can teach myself to read much better if I can hear the words in my head. The tapes with the book are fine for that, with good, clear, slow pronunciation, but they're not good for learning to speak, because, again, they don't have the appropriate sort of drills.
So, in summary, it's quite good for learning to read, so-so for learning to write, and useful only as a secondary resource for learning to speak.