I agree with gmatninja as far as giving weight to non-official GMAT tests. Here I believe are the reasons why almost all of the official GMAT questions are of significantly higher quality than non-official GMAT questions(okay GMAT test writers also make rare mistakes, they are humans after all):
1) The questions are written by some very bright people and it is a team of people. Each question is thoroughly checked for ambiguity, language, and style.
2) The use of experimental questions during the actual GMAT exam helps the test writers to further eliminate questions that contain any form of ambiguity. For example, if significantly more men than women get a particular question right, they eliminate it. The stressed test takers are their free guinea pigs!
3) The test writers spend a lot of time generating "distractors" which are wrong answer choices that lure test takers. This is a standard technique for multiple choice questions for generating difficult questions. The American Math Competitions also employ this strategy, this is another exam that has an extremely high quality of questions, perhaps better than GMAT in my opinion, but I digress.
4) The GMAT has a very specific set of guidelines of how questions are structured, I have a sense of what that is, unfortunately I cannot explain it in words(okay I can try but that would be a long post). It is almost as if they have a set of binders in their office that categorizes all of the ways they test specific ideas and how they want writers to write new questions. You can only develop a sense of this style if you use your precious time exclusively on official GMAT questions. This is why if you spend all your time on non-official GMAT questions, then there is typical adjustment shock.
5) At least on the quant, I know that many questions are structured in a way(especially the difficult ones), where on first reading one tends to go in the opposite direction of where one actually needs to go. It happens to me often. This is an intentional design of these questions, and it is natural for most of us to do that, however what matters is how quickly you can detect the correct path.
In a nut shell, majority of the non-official GMAT questions aren't able to live up to the above set of requirements and you often see people flouting these norms in the following ways:
1) Tedious and complicated algebra.
2) Questions out of scope: example an ellipse question I saw in some GMAT iphone App.
3) Poorly written long question stems that are full of ambiguities.
The only way I have seen non-GMAT questions overcome this is by literally copying an official GMAT question and just changing the numbers slightly. This is done to different degrees of sophistication. Some companies have literally copied the official GMAT questions and changed the name of people or the numbers. Others are a bit more sophisticated. But I think this is okay because GMAT test writers themselves lift questions from other sources, I have seen instances from AMC, and a recent question I had on the exam that was straight from a GRE Math subject test.
In a nutshell, stick to Official GMAT questions and Official GMAT practice tests, unless you have exhausted them all.
New!2016 OFFICIAL GUIDE FOR GMAT REVIEW: Free Video Explanations.