I can't say much about the Princeton classes. I had signed up for their in-class preperation course but it was cancelled. So I went with the Kaplan online course instead and it was great. They provide you with a couple of preparation books which you can use for your homework or for independent study. The course itself was very helpful. We did a complete math review for the first couple of classes and then worked on vocabulary and the writing part (which were especially useful for me because English is my second language). I think Kaplan says that you can raise your score by 200 points (old score). My instructor even said raising the score by 200 points in each section is possible. They also give you the option of repeating the course for free if you're not satisfied with the results.
I was able to raise my score by more than 200 (in total). I think the most helpful tool Kaplan offered were the many practice tests and their analysis (what part of math/vocab you still have to work on). You could even send in practice essays that would get scored by Kaplan people who also made suggestions on how to improve your writing.
Those courses are expensive but if you hit a score limit on how high you can get on your own, I think it's worth the investment.