As with any personality test, it should not be taken too seriously, especially since it was written by two laypeople. It can, however, be used as a rough indicator of some of your obviously strong personality traits. In any of the categories in which you score at one extreme or the other, chances are that you and others will be able to percieve this in your behavior.
For example, I am almost entirely a P, one of the traits of which is the extreme dislike of being tied down to a schedule. P people like options, and lots of them, and they don't like making a decision until they absolutely have to. This pretty accurately describes me most of the time. I may or may not be in the right mood to do things, so why bother scheduling them?
My fiance is a J; J's like everything nice and neat and organized in the temporal sense; they schedule everything. My fiance has a little hp palmtop that he has set alarms in to remind him to go to class, as well as any other event that he has scheduled.
I find the first letter of the test, which indicates if one is an introvert/extrovert, to be not that useful, since this is pretty easy to tell anyways. Any letters in which you are 60/40 in, or even 70/30, I wouldn't pay attention to - you don't register enough in one category for the test to say anything definitive about you.