I am certified. So obviously, I am biased and felt the fee and the time and trouble was worth it. When I took the test, the state I was working in had a state certification, which I had, and I still felt the NBCSN certification was worth the investment. I did prepare - studying questions, did some reading in areas that I had little personal experience in. I did purchase a prep "course" but actually never really spent much time with it. It was put out by Mary Kornbluth (I think I spelled her name right). I have heard that some states offer prep courses, and there is a prep course at the NASN annual conference the day before the certification exam is administered. This year the conference is in Cincinnati in June.
I think in school nursing, where our bosses frequently are not nurses, and have little or no idea what makes one competent, certification is a way to validate that you have the knowledge and foundation to perform the job.