I have the Princeton Review set and it seems like its pretty good material, at least with regard to test taking strategy. I also do at least 1 MCAT question of the day every day. For the stuff I don't know, I will often end up spending 15 minutes researching why the correct answer is the correct answer. I'm ~ 3 years away from my MCAT so at an average of 15 minutes per day that gives me an additional ~ 270 prep hours. I still plan to do several months of intense studying for 8 - 10 hrs per day, but the prep hours leading up to that time, taken in 15 minute chunks, should be quite helpful.