I personally improved 24 points from first PT to test day taking a Kaplan class, and do not think I could have done so studying on my own. It's not just the motivational factor that most people cite in support of classes - it's the value of having an instructor. It's the same reason, I think, that going to a school is superior to homeschooling. Having the feedback of others, and especially of an experienced instructor, will always trump going it alone.