I work as a career consultant for an outplacement company, and am currently studying Organizational Psychology towards a PhD.
The Myers Briggs (MBTI) assessment is one of many different assessments available to measure personality traits, and other traits that can be useful in learning to work well with others, or to understand yourself better, or to guide career choices.
The MBTI has a very mixed response among psychologists, because its validity and reliability are not as high as other assessments. It can be useful in understanding the differences among your colleagues. It can help in finding new ways to relate to your manager or others at your workplace.
I never recommend to clients that they make a career change decision based on any one assessment. If a set of different assessments are done, then you may have enough information to guide a career decision. Assessments such as the Strong Interest Inventory are designed to be used to guide career decisions.
Another assessment that is readily available an inexpensive that I often recommend to clients is Strengthfinder 2.0 (www.strengthsfinder.com). You don't have to read the book, but you need to code that comes with the book to take the online assessment. The results will tell you about your natural talents and give you information about how to use those to your benefit in your life and work. Many of my clients have been very pleased with the accuracy and amount of information this has given them.
I made a career change after taking the Strengthsfinder assessment, but only because I already knew that I was in the wrong job and I was failing at it!