My understanding is that students meet in small groups and are presented with case studies. Collectively, they solve them and recommend a course of action with an advisor who chimes in with advice to keep them on track...
so for PBL based schools, it really depends on whether you learn best in a lecture-based setting, or a small group setting. Lecture based schools are exemplified by schools like P&S and UCSD, whereas, Cornell and Northwestern are known for their more progressive curricula.
Characteristics of PBL...done well
They'll give you specific case studies. For example, they'll say that a patient comes in with chest pains, high bp, etc. Then you work together to try to find out what the problem is, order tests, etc...through it all, a lot of time is used to research the physiology, biochemistry, genetics, etc. of the disease...this is all guided and facilitated by the instructor and the handouts...
Strengths of PBL:
Small group learning, led by 1/2 instructors - get to know students/faculty in a small group setting
Incorporates group learning because each member is responsible for a discreet body of knowledge
Shortfalls of PBL:
A lot of people don't like PBL because they think it's a waste of their time. Whereas you're responsible for a clearly defined set of information in lectures, PBL can often lead to tangents and cause you to research information that you feel is irrelevant, i.e. lack of direction arguement. If you are a gunner/loner, you probably won't like PBL.
I don't think I would like too much of PBL because I really enjoy learning new material in lectures from people who know a lot more about the topic than I would if I researched it on my own for a couple of hours. When I do read about the information, than it can help to solidify what I learned in lecture; reading is a lot more helpful for me when I already have some familiarity with the subject. I do aknowledge that there is sooooo much information to be learned in the pre-clinical years that the lectures will not cover a lot of it and they may not be that helpful, but I remember being quite surprised in undergrad a couple of times when I hadn't studied on my own at all but I did have a pretty solid grasp of the material just based on lecture. I think small group PBL sessions would be helpful as a supplement to my education, helping to solidify what I have already learned, but not as the core of my education.
Cornell, NW, and the UCSF-UCB program wouldn't be good places for me. I'm just glad I was too lazy to fill out those secondaries. I'm learning so much more as I go through this process than I did when I started it. That's why I think it's a good idea to apply to quite a few schools (esp for Cali residents) because, as with other things in life, you learn so much more when you go through the process than when you read about it.