Clients are often interested in psychometric tests but not many understand what they are for. Many feel quite sceptical about them, whereas others trust them too much. Broadly speaking, there are three different types of psychometric test:
Aptitude tests look at your strengths and weaknesses as an employee. Aptitude tests usually focus on verbal skills, numerical reasoning and spatial awareness. Some jobs may have industry-specific tests such as creativity tests.
These tests match your personality to a career direction. Questionnaires look at aspects of character which can help to identify the environment in which you are most likely to succeed. Use the NEO-PI (or other measures of the Big 5 personality traits) for best results (and not MBTI)!
These tests (attempt to) match your answers to the career path (or paths) that interest you or which suit your style the best. Can be useful but not by themselves. Handle with care. You are a human not a type.
These assessments all have limitations. However, the better tests can be effective at helping to ensure that career decisions are made as objectively and with as much self awareness as possible. Tests must have good reliability and validity.
Further information on psychometric tests can be found at the BPS page on 'Psychological Testing: A User's Guide'.
Without doubt the most comprehensive list of psychometric tests I've seen is by my fellow Occ Psych Mark Parkinson. I couldn't hope to better this list, so here it is.