If you take these tests honestly, and in private, they can tell you a lot about yourself. However, these tests are very easy to mess with to make it look like someone you are not. Once you realize what they measure and how they measure it, it is very easy give the "right" answers for someone who is looking for that special INTJ or whatever they're seeking.
Also note that on some days you may find that your personality swings one way, and on other days your personality may swing another. It is but one very precursory self evaluation tool. I would not assign much significance to such tests. It is about as useful as the SAT is to showing how well you will do in college (and the answer to that question is that it is damned near worthless).