I took the Princeton course this past summer and I am pretty divided about its usefulness. I must say, however, that from the first "diagnostic" I took, my actual MCAT score did not improve. This itself, would suggest that the course did not help.
Did it help me stay motivated to study? For sure! Having regular classes to go to definitely helps you want to stay on track and keep up with the material, but I think this is really all it was good for, and even so - to a very limited extent.
The material that they provide you with is fantastic as well. It's a great detailed overview of everything you would possibly need to know. BUT...this is just where the problem is
Princeton is a good course if you've been out of university and away from the sciences for too long because they provide you with the necessary details and explanations about certain concepts. However, for an undergrad, I don't think that a review of the material is what will benefit you.
The MCAT is structured so that you actually need VERY little science background. Although the background helps, I don't feel its essential. All the information you need to answer questions will be there in the passage (aside from the free-standing questions - of which there are few - and these can be answered from your general science knowledge anyways). So, having said that - if all the information is there anyways, then what you really need practice on, and what Princeton does not provide, is an understanding of how to answer the questions and how to tackle the passage. Namely, you need to get really good at strategizing. I can tell you for sure, that spending hours in a class will not do this for you. What you need is practice, practice practice. I would suggest buying the materials off of somebody and then studying on your own.
One definite good strategy is to buy the AAMC practice exams. These are very indicative of the real MCAT. and again...PRACTICE PRACTICE PRACTICE!!
says thanks to SciMania for this post.