about half of all test takers took the osler course last month when I took the boards. Not sure if their pass rate is significantly higher than those that did not take the course. I'm sure it is useful but not critical by any stretch.
I agree with above posters that it is a strange test indeed, and difficult to assess weather or not you did well after finishing unlike written tests. Having a systematic way of approaching cases and practicing talking things through with colleagues is useful, but at the end of the day, still a difficult test to both prepare for and to take.