Sorry to resurrect an old topic, but I searched for this question online and found this forum. You guys helped me, so I'd like to help others by relating my experience.
I started studying for the GRE all the way back in 2012. I always thought I was good in math, so I hadn't bothered to study math at all for about 9 months. Then, I tried a practice test with manhattan prep and found out that my verbal was just fine, somewhere around 164, but my math was horrible, at 155 (and I'm an engineer, wtf?) So I started studying math very diligently. I studied much of Kaplan, and learned a few tricks, and then tried a test from ETS PowerPrep II. I scored a 159. It looked like I hadn't improved at all. Next, I took a math course from Kaplan specifically designed to get an "elite" score. I thought I was doing pretty good. I also studied Princeton Review, took their math tests, as well as many tests from Mahattan Prep. My scores were varied, and sure enough, one week before my actual test, I took the second ETS PowerPrep II test and scored a 162 in both Quant and Verbal. This was pretty good, but there were questions on that test that had me sitting there for far too long trying to figure out, and I wanted to make the "average" score or better for successful applicants into MIT, Stanford, etc.
This is where Magoosh comes in. Knowing I needed something drastic, and only one week before the test, I bought the Magoosh package for $100 and studied for a solid week, (including a full weekend). At first, the Magoosh problems would stump me. But after a little bit of time watching their explanatory videos, taking notes on problems that stumped me the most, and all that practice, I definitely improved my speed, comprehension, and accuracy. Here are my stats:
Kaplan: (unsure, the course expired, but I recall the average being somewhere around 161 for quant and 163 for verbal)
Mahattan Prep Average: Q159, V161
Princeton Review: Q163-164, V:163
Magoosh Score Predictor: Q158-163, V155-160
PowerPrep II first test: Q159, V163
PowerPrep II second test: Q162, V162
Actual Test: Q166, V164