They can be very useful, but I assume some are probably better than others. I attended a 5-day camp in the Poconos that was very good at teaching the PMBOK view of the world. However, I was disappointed at the way the exercises were conducted. The course author seemed to think that ‘buying a house’ was an appropriate theme on which to base exercises illustrating project management principles – I think they could have done much better than that and used a better model. "Constructing a building" may be the most overused scenario in explaining project management, but it would have served us better in this case.
Nevertheless, I did gain a good understanding of how to apply PMBOK principles (and what to expect in the exam). I also read as many books as I could get my hands on and used a couple of computer-based practice tests. Finally, I took a week off before the exam to settle down and clear my mind, and passed it on my first try.