What attracted me to personality typologies in the first place was the prospect of gaining some understanding of myself. But, I have to say that I have been very dissappointed; surely I understood to some extent what the types were, especially with regard to introversion and extraversion, yet these concepts have not helped me at all in what I was initially seeking. They have been even less helpful with regard to other people; typology has been useless to try to get closer to others by trying to understand them.
The types, as abstract concepts that they are, merely summarize some patterns that can be discerned in large groups of people, such as many patients who went to see Dr. Jung. However, nothing guarantees that someone who fits, accoring to the test, to some type will behave anywhere near what the description presents as his personality. Since people seldom deal with large numbers, but with small numbers, or in reflection with just one, then these patterns are pretty much useless.
Moreover, the types could be thought of not as patterns observed in actual people, but simply as yet another expression of those things that fascinated Jung, namely the archetypes. Jung, I read, considered them as comparable to Kant's categories, that is, that they are without any empirical content and that precede any experience. So, in fact, when saying, "he is an INTJ", or "she is an INFP", you may not be asserting anything about him, or her, but merely porjecting one or another archetype unto them. Now as we become more conscious and more individual, such a view of reality fades away. I think this is good, since it allows you to SEE what is in front of you.
Anyway, just some sudden burst of thoughts...