The main issue with Kaplan is that they do not use real, licensed LSAT questions. As a result, some other questions are just off. It's been awhile, so I suppose this could have changed, but that was the biggest legitimate complaint back in the day. The difference is real.
I did Kaplan's self-study books in 2012 - it was 100% real LSAT questions. I don't know anything about the actual Kaplan courses though.
Princeton Review's self-study books were 100% fake questions.
I also did the Powerscore books. Of the three, I liked Kaplan the best. Princeton Review was a complete waste of time and Powerscore broke things up just way too much into subcategories - it became more so about learning about powerscore's method than it was about learning how to take the test.