I'm currently a senior in high school. I have been in several classes which used the "problem-based learning" approach. Some of these classes worked very well and others didn't.
Biology Class - the Forest Preserve Project
This project was from a biology class. The class was supposed to be an environmental research group hired by a city to answer the question: Should a housing development be allowed next to a forest preserve? It was supposed to take two weeks, which is really not enough time. A couple months later we were so far off subject that we were researching the tax base in nearby suburbs, calculating how much the city would have to spend on road resurfacing, and determining how much pesticides the average golf course uses in a year. By then we were tired of the project and annoyed at the class.
Physics - Astrophysics and Modern Physics
At the Illinois Math and Science Academy, there are some physics electives that are almost entirely problem-based. The best two are Modern Physics and Astrophysics. At the beginning of the semester, the class does a project like: Find six pictures taken by the Hubble Space Telescope, think of some questions about them, and make a Web page with your pictures and questions. After that, people choose something they are interested in and work together in groups. Every so often groups will make a Web page to show the rest of the class what they are doing. Here is my group's Hubble Space Telescope Page. I liked these classes and I learned a lot.
What's the difference?
Size of Problem: The biology class project tried to do too many things. We weren't supposed to care about how much tax money the city would make; we should have stuck to the environmental part. The problem became too big and complicated for the class to understand, so students weren't interested in it anymore. IMSA's physics electives on the other hand stick to parts of large problems and questions that can be realistically answered in a reasonable amount of time. That way students can actually see some progress and the project doesn't slow down to the point where nothing ever gets accomplished.
Access to Information: The class doing the forest preserve project had no access to information at school. IMSA has Internet access, so when they tell us to go out and find something, we can. On the other hand, the biology project required some hard-to-get information, and some students ended up driving 20 minutes to a big library to find it. This makes students mad, because if they spend an entire evening looking for some information but don't find any, they've wasted a lot of time.
Organization: IMSA class groups are usually three people. The whole class never decides on something as a whole. The biology class worked on the project as one big group, which would have been fine if it had been organized. The teacher asked "What don't we know?" and wrote everything we said on the board. Some kids didn't care and just said anything, and then we had to go and research every single thing anyone said. This is how it got out of hand, and how everyone started to feel like the project was pointless and we weren't getting anywhere. In Modern Physics, questions not really having anything to do with the project were remembered as something for possible future projects, but they didn't detract from thinking about the original question.