Alexleung:
I need to make a full posting on this some time soon but there are two kinds of difficulty on the GMAT Quant.
One type is the "upfront difficulty" dealing with concepts, formulas, etc. This is the difficulty that most people are scared of and I think the kind that Night Reader is talking about. I would agree that this difficulty is lacking in the OG 11th and 12th. You will need additional sources for questions that have more difficulty in concepts and in formulas, number of calculations, and complexity.
There is a second type of difficulty however, and that type is what I call "sneaky difficulty." It is tough to say that none of the questions in the Official Guide are over 700 level because difficulty is not only what you can see, but the tricks that are built in. As long as lots of people miss the question, for whatever reason, even if it would be considered a simpler question, then it could still be a 700 level question. Sneaky difficulty is what the OG does better than any other source.
So, what you should focus on with the Official Guide is avoiding mistakes, assumptions, etc. For test day you need to make sure that you are able to avoid mistakes that the GMAT pushes you into like answering the wrong question or taking the wrong strategy.
For the "upfront difficulty" you will need other sources as Night Reader said this not addressed as well on the GMAT.
Hope that helps!
