Based on what I have read on this and other forums, the OG is actually a pretty fair representation of the material (quant and verbal) that you see on the actual test. However, once you hit questions above 680 level or so, they do get harder than most of the questions you see in the OG, which is why a lot of people turn to Kaplan800 and MGMAT tests.
I have tried 5 MGMAT tests now, and would have to say that I found a number of their problems on the quant to be pretty difficult. The questions did not necessarily test for harder concepts; they just twisted them all together in a way that really required working the problem.
Their verbal, on the other hand, has some questionable CR questions. Some of them involve math, which is not true of the actual test. Also, a number of them are much harder than what you will see on the test because of the sheer number of details you have to keep in mind. HOWEVER, this is still great because you get used to really difficult verbal questions. I have to say that MGMAT tests really improved my ability on the science passage. I used to hit 1 out of 4 questions on these passages. Now, I hit all or 3 out of 4.
