Professor's Dave GMAT Advantage is fantastic. I am about 1/4 of the way through it.
It helps you understand the differences in easy, medium and hard problems.
I've taken a break from it and started using
Manhattan GMAT books which just talk about a specific topic, which has been helping me brush up on some areas that I need help in.
But as soon as I am done with the
Manhattan GMAT, I will go back to GMAT Advantage.
Any other questions, don't hesitate to contact me.
Sincerely,
wiseguy