I will stay neutral since I am a biased party and one of the people who created the tests. There are many testimonials on the site in terms of the usefulness/scope/staying within the limits.
In my opinion they are within the scope of the GMAT though the actual boundaries are usually debatable to a degree. The part where they may be going out is difficulty of some of the calculations (a few of them require brutal multiplication/division) but they have a point too in the sense that you will always encounter on the GMAT something that you will not know how to solve and need to make a decision (spend time on this or guess and move on). Most high-scoring people refuse to guess.
There are a few questions that do go outside of the GMAT borders but there is usually a reason for that - e.g. teaching a concept or shortcut that can be used on questions within the testing realm and again to simulate some of the "experimental" question nature where you may presented with a non-orthodox question that you have never seen before.
Anyway, I would do a search though and see this one result-correlation-between-gmat-and-gmat-club-s-tests-30989.html
The thread is a bit old and not very representative as we have been doing revisions to the tests over the last few years. One major revision done in 2009 and one currently under way (we look at the results and make sure the questions are properly tuned - e.g. no confusing wording or multiple solutions, etc.).
P.S. Sorry for a long note. Was going to be a 2 sentence reply
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