Unless you plan on preparing for the GMAT in under a week with nothing but the free online materials, you'll need to buy the Official Guide. It's worth having for a couple of reasons:

• All of the questions in it have been used on real past tests; they're official in the sense that they're created by GMAC, the same company that makes the GMAT
• It has about 900 questions, more than enough for many people

On the other hand, the Official Guide has its flaws. It contains next to no strategy advice, the question explanations are often next to useless, the Math Review is very dry, and if you take the book's advice and start by doing the Diagnostic Test, you might give up on the GMAT before you've even started.

Having said that, it's one of the biggest sources of official practice questions you can find, and if you don't have space to carry it around with you, you can access all of the same questions online.

It's a good idea to try some of the questions online anyway, as that's how you'll sit the test. Another benefit is that you see how long you've spent on each of the practice questions (I talk about timings more under Pacing in Section SIX ~ Organising your work).

Think of the Official Guide as your bible. Get to know as many of the questions as possible inside out, but feel free to ignore everything other than the practice questions*. Use the questions to work out whether you need help, and if so what with. But any other resources that you use should be chosen to provide targeted help either because you've had difficulty answering a certain type of question or because you don't fully understand part of the underlying theory.

*The new introduction to the Sentence Correction section is worth reading