Since I read this thread so many times leading up to my test, I feel like I have to share my data and experience...thanks to all of you that posted before!
I used Magoosh almost exclusively and answered most of their practice questions over the course of roughly 4 weeks. I ended up taking 3 Magoosh practice tests, the free Manhattan Prep test and both Power Preps.
1) 156 Q 162 V - Magoosh before studying at all
2) 161 Q 164 V - Magoosh 16 days before test
3) 164 Q 162 V - 1st Power Prep 12 days before test
4) 163 Q 162 V - Magoosh 1 week before test
5) 159 Q 163 V - Mahattan free test 5 days before test
6) 166 Q 167 V - 2nd Power Prep 2 days before test
162-166 Q 160-165 V - Magoosh Predictor
165 Q 166 V - Actual
I'm pretty happy with my score, although I know I made at least one stupid mistake on the first quant section and realized it right after my time was up. I will be applying to Computer Science programs, so I would gladly trade a few verbal points for a higher quant score.
Overall, I thought the Power Prep tests were slightly easier, yet very close to the real thing (perhaps the difference was just nerves on test day). Magoosh was generally a bit harder in Math, but their video explanations were really good refreshers since I took Algebra 2 way back in 96-97, and I thought that their verbal and quant questions were both similar in style to the actual test. I didn't think the Manhattan questions were very representative of the actual GRE questions and wouldn't recommend it.