First thing to realize is that The Good Schools Guide should really be titled "The Good Private Schools Guide with a few State schools in it as extra filler". If you're looking for information about State schools, the problem is that their coverage outside of London is very limited (it's not too bad for London, probably reflecting the location and biases of the editors'). For that reason, I wouldn't really recommend it for people looking to find out about the state schools in their area.
Second thing: the editing is inconsistent, and some editors have a degree of preference for specific schools. I don't want to use the word bias - that's taking things a bit too far - but realize that the editors' are generally parents themselves, and in their reviews they often reflect their own educational preferences. The fact that there are many editors (how else would you cover the whole country!) is what causes the inconsistency.
Third thing: changes between editions are actually. We have both the 2007 and 2011 editions, and apart from noting changes of headmaster and the addition of a small number of new schools, most of the reviews are verbatim identical in both editions. This is surprising as a school can change considerably over four years. The new book looks thicker and much more impressive than the older one, but that's mainly the result of using a larger typeface!
With all of the above caveats, *if* you are looking for private schools and *if* you don't have the book already, I know of no other resource out there which is anywhere near as comprehensive in its scope. It will help you to put together a shortlist of schools to visit, and it will help you to think about what to look for and what questions to ask. It is not a substitute for putting your shoes on and making lots of visits, however, and you probably will not agree with the editors in many cases (I certainly didn't).
If you're new to looking for a private school (as my wife and I were), it is useful to know that there is much more variation in ethos and approach in the private sector than you will find in the state sector - in other words, you have a very real choice about the type of education your child will receive. Academic, sporting, pastoral and disciplinary approaches very considerably between schools, and this means that with some effort you can really find a school which will provide the best environment for your child. The GSG can help you to get a feel for some of this variation, but it is critical to use the visits you make to work out exactly what it is that you are looking for.
Final caveat: the schools in the GSG are, indeed, "Good". Many schools which are not in the guide are also good, and deserve consideration as well. If you consider the GSG as no more than a starting point in your journey to find the right private school for your child, it will serve you well.