Yes, the test did get easier after the first 20. I must admit that the first few questions were slightly harder than I was accustomed to facing that early in the test. I took way too much time just staring at the screen hysterically (even on easy questions). I wasn't sure though if the questions got easier after I went to the bathroom at question 18 or so because I had missed a few out of anxiety, or because this was the format. The reason it's a little difficult for me to say is because I had not done as well as I typically do, and of course questions get more difficult the better you perform.
The reason I say that the material was representative of what I saw on PP is because I would have expected no better than what I scored if I'd turned in a similarly inadequate performance on the PP tests. I found the verbal section to be slightly easier than the PP tests but of course that's the luck of the draw. The point is that it's very similar. Unfortunately I did do some of the questions in the OG before I took the PP exams, but not enough that I really recognized more than one question per section. I think the power prep tests are also tremendously useful because they give the test taker the opportunity to see how many they can afford to miss and still get a particular score. This helped me guess that I was around 700 before I chose not to cancel my scores.