I took a 12 week (I think) course that was held at my local homebrew store, taught by a Grand Master judge, to prep for the course, and loved it. Even if you aren't planning on taking the test, a class like that will help you in every aspect of tasting/evaluating and brewing beer.
I am Certified and scored high enough to test for National, though I am not sure when/if I will. I don't judge enough competitions at this point in my life to make that extra effort worth it, but I may down the road.
I would recommend becoming a judge to anyone that loves homebrewing and beer in general. The comps are fun, the people are awesome and you a really able to help fellow brewers by providing constructive, unbiased evaluations of their beer.