In 4th grade we have different examples of project-based learning. The opera, of course, is a huge project that the students work on. The students write, produce, and perform a "musical" while applying different concepts they have previously learned. Likewise, in our nutrition unit the kids research different nutrients, and then present the nutrient to their classmates. The kids were then asked to develop a balanced meal, and present it to a group of "guests". The guests asked the students questions regarding the nutrients in their food, and why did they decide to "pair it" with the other food in the meal.
One of the biggest problems I see is that not every kid puts the same effort on their projects. Making sure that everyone is working (if it's a group project) and that they are all doing their best is very important.