I was reading a thread on a GMAT forum a few days ago, and saw that the owner of a test-prep firm was telling students that they should avoid independent GMAT tutors who rely on commercially available GMAT materials. The owner's logic sounded good (hey, look, we know enough about the GMAT to write our own book! hire us!), but I wholeheartedly disagree with the idea that you should use GMAT materials written by random test-prep companies. The only GMAT materials you should completely rely on are the GMAT official guides--which are, of course, commercially available. Anybody who claims that they've written something better is either lying, or completely deluded about the complexity of the GMAT. It is extraordinarily difficult to copy the writing style of the GMAT, even on math questions that contain few, if any, words. The GMAT is a ridiculously complex test, which is exactly why I love it. (And, perhaps, exactly why you hate it.) I don't know exactly how many brilliant test-writers are employed by GMAC, but I suspect that it's a fairly large fleet of people, and I'm certain that questions are very thoroughly vetted before they are ever inserted into the scoring algorithm for the real test.
The challenge for "knockoff" GMAT writers is even greater on the verbal section. I've written and edited TOEFL practice tests, and I can say that took an enormous amount of effort to make our material sound even vaguely like the real thing. GMAT verbal questions are even more detailed than TOEFL questions, and it's extraordinarily hard to write GMAT questions that will be "hard in the same ways" as the real thing. I've found countless mistakes in widely-published GMAT verbal material, and the companies seem to struggle with all three verbal question types. Frankly, most of the mass-market GMAT test-prep publications will do more harm than good if you're using them as your primary verbal prep resources.
A majority of my students contact me after they've already taken either a GMAT prep course, or a reasonably thorough self-study course. I've seen refugees from Veritas, Manhattan, and Kaplan in roughly equal numbers, as well as an occasional graduate of Princeton Review or other (less well-known) competitors. Some of these GMAT companies are better than others (and one, in my opinion, stands head and shoulders above the rest), but the bottom line is that all of them are inevitably limited in their ability to replicate the GMAT experience.
For what it's worth, the company that makes the very best "knockoff" GMAT materials manages to capture about 80-90% of the real GMAT experience, and that's an astounding achievement. (I feel that it would be inappropriate to make an endorsement here, so I won't name the company.) This same company probably makes its verbal questions look like the actual GMAT only 70% of the time--it's amazing how often I see (or hear of) real GMAT questions that fall outside the scope of even the best "knockoff" curriculum. Again, I think that this company is absolutely amazing for doing as well as they do, but that shows how hard the task of "reproducing" the GMAT really is.
The other GMAT test-prep companies? Not even close, and I've looked at material from some of the most expensive, "boutique" test-prep firms, as well as the household names. As a general rule, the "knockoff" verbal questions will do more harm than good, and even the second-best large company produces GMAT tests that are riddled with typos and errors.
I mean none of this disrespectfully--quite the opposite, actually. These firms throw a ton of resources into an extremely difficult task, and their results are necessarily mixed. But before you buy a GMAT practice resource sold by a small company, be very aware that the material will probably be filled with inaccuracies. There are presumably a few gems out there, but all of them should be handled with extreme care.
I'm regularly asked if I include some sort of textbook or workbook or self-published GMAT guide as the backbone of my tutoring program. I write supplementary GMAT questions all the time, but trust me on this: you probably wouldn't want to hire me if I relied primarily on a self-published resource. A good GMAT tutor or test-prep firm will supplement lessons with their own materials, but any independent tutor who says that they've written a magical, best-in-the-industry GMAT resource is probably exaggerating, if not egregiously lying.