It is all in how you use it. It is an aid, not a be-all, end-all indicator. People have tendencies, and personality is not set in concrete. We change over time and with experiences. What the Myers-Briggs helps with is indicating how people will interact with each other and how they will perceive situations. Humans are by nature complex. There is nothing out there that will tell you without doubt what a person is really like or how they will act. What these assessments give you are tendencies and proclivities. This is why there are many different questionnaires for personality assessment, some more in depth and some way too shallow. Most people are familiar with a very short and superficial Myers-Briggs questionnaire, which is less than optimal. A more in depth questionnaire along with observation in real life situations can be very informative in understanding how a person is likely to work with others and deal with situations. It shows tendency and likelihood, not a hard prediction. I dislike questionnaires in general because people who know they are being assessed will change their answers every time they take it and try to second guess every question. They are problematic.

The 5 factor model is:

Openness-to new experiences

Conscientiousness-attention to detail and consistency

Extraversion-a scale between introversion and extraversion

Agreeableness-how easy they are to get along with

Neuroticism-tendency toward negative views and emotions

There is overlap between the models, and these are not the only 2 theories out there. They do have explanatory power in assessing people and their behaviors, but as I said, personality is not set in concrete. You grow and learn over a lifetime, recent experience can have a huge impact on a test result. It is more accurate to observe a person in real life over an extended period, but if they know you are assessing them, they do change their behavior. In both models, each factor is a scale. They do not show an either/or category. You need to understand where on that scale they fall.

Any assessment of people is problematic and will not give precise measures. People are complex, and are made up of their biology, their experiences, social environment, current situation, beliefs, and many other factors. If you are completely honest with yourself, you can assess yourself. This will help greatly in understanding personality in other people.