Just wanted to add my review to the list. I got the GMATFix Official Gude Companion recently as well.
There are actually 2 versions. gmatfix.com advertises the online version, but if you ask for a hard-copy, you will be sent one (more expensive, but you may prefer it). I received a hard copy of the book, so I have access to both online and in print.
As Ascara pointed out, the online version was a bit cumbersome to work with. However, last week the website changed the display and it's now much smoother/easier. The new application uses the iPaper module to display the book. I've had no problems.
As far as the book itself is concerned, I think that it offers something for all scoring levels. Whether you are a 700+ or a 300 scorer, you need to know the basics and you need to know the different patterns that come through the GMAT. This book excels at showing different ways to solve the same question. I actually disagree with Ascara above as I think that as the questions becomes more difficult, the alternative solutions and detailed explanations and strategies become more helpful.
Something that the other reviews have not mentioned (I don't think) is the Advanced Speed Drills section. This section is specifically for high scorers. As a matter of fact, if you're scoring below the 65th percentile in math, you proabably should study the rest of the book and leave these advanced drills alone.
In data sufficiency, the solutions are a bit different. Instead of showing multiple ways of solving, every DS problem is broken down into 5 steps
(1) Rephrase Q. (2) Anticipate data (3) Rephrase/Evaluate Easier statement (4) Rephrase/Evaluate Harder statement (5) Combine statements if necessary.
Within these steps, some solutions show you two ways to evaluate the same statement (for example, algebraically or by trial & error). Almost all questions (DS and PS) are followed by specific lessons (Take-Aways) that should be drawn from that question.
The OG Companion is large (600pgs), so if you buy the printed copy, it can be a bit heavy to carry around, especially if you also have the OG. All in all though, I would say it's an excellent resource, but incomplete (there is no verbal).
On the company site
www.gmatfix.com, there is a preview that gives you an idea of the different sections and of the writing style.
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