Phil, I've taken and passed Installer I, II, and Designer I tests. Installer I will test your knowledge on NEC codes, OSHA regulations, connector and wire identification, construction practices that affect your job (basically structural stuff), and more of the like... as deco said, the one-day course will be all you need to pass the test, along with your 6 years of experience. They'll provide you with a study guide that you'll be able to take with you and study overnight. Really, it's a breeze. If I remember, it's 100 questions, and you get 3 hours (I think) to finish. Plus they mail you a nice little certificate to stick on your wall when you pass.

Hopefully you won't stick it up with a thumbtack.