Well...you learn a lot more indepth and it helps you think a bit more critically with a broader understanding of issues.
For me, it was also a personal challenge to take the class (it was 12 weeks long and excellent)and pass the blasted exam.
The extra money earned isn't bad either.
My hospital is paying us for the 12 week long class to become certified, so I took advantage of it. Even though I have been a nurse for a long time, it's nice to have more indepth knowledge.
BTW, one co worker took the ANCC and the rest of us took the AMSN. The ANCC focused more on ethics, law, etc..from what she said. For us the AMSN was strictly all the systems/diseases, labs, and a lot less ethics.