My friend said it was really effective - the person running the test told him a bunch of stuff about his personality and management style based on the test that he hadn't explicitly thought about but was helpful. It can help point out your blind spots and how you should approach people with different personalities and styles.
The problem with the Myers-Briggs personality test is that it is codswallop. It is made-up. It is not supported by scientific evidence. It has no predictive value. It is meaningless.