The book is intended for use alongside a Greek New Testament (preferably a reader's edition that gives a translation for uncommon words at the bottom of the page) and a lexicon, although most New Testaments have a functional lexicon in the back. Whereas those other resources are helpful for figuring out what a word may mean, the syntax guide helps readers understand how the grammar and sentence-level structure determine meaning. Just as a foreign exchange student studying English might understand each of the words in the phrase "in a pinch" without grasping the meaning (in a difficult situation), so too a student of New Testament Greek may have an idea of what all the words mean without understanding how they relate to each other to make meaning.
The best way to evaluate a resources like this is to use it. I had A Syntax Guide out on the table with my Reader's Greek New Testament and my Logos iPhone app to help me read through Galatians. I found that prepositional phrases tended to cause me the most trouble, and notes like "διὰ + interval of time = after," "ἐν ἐμοὶ = in my case" and "ὁ δὲ μεσίτης ἑνὸς οὐκ ἔστιν = now a mediator is not for one party only" made reading easier. Other times word order or a pronoun for which I couldn't place the referent noun was throwing me off, but A Syntax Guide set me back on track. Sometimes it's the simple things that slow me down.
I admit that I'm not very advanced when it comes to reading my Greek New Testament. I'm pretty dependent on external resources like Bible software or a good English translation. A Syntax Guide is a nice tool to have. It's not essential, but in tandem with other resources, it's proven valuable to my own studies.