I did an intensive course (Kaplan) and my score improved around 10 points in six weeks. And that's after I had already taken a previous prep course and ended up scoring lower than my diagnostic, my first time around. Not saying it's necessarily worth the absurd amount of money money, but for me it was. They eliminate distractions and supply the tools you need to do well.
Can't speak to how it worked for the other people in the course. Although, of those that I spoke to, it seems like it helps the people who were scoring low the most (20-27ish). Most were able to bring it up to the 30's. Those who started with 30 and wanted 35+ didn't find as much benefit.