The main strength of IQ tests is the ability to provide a rough estimate of the raw intellectual ability of a test taker. The results of tests can be used to estimate future performance of test takers, but IQ tests are limited in scope and therefore are not always good predictors of academic or job performance.
One weakness of IQ tests is that designing a test to interpret intelligence presumes that intelligence can be adequately defined and tested by using a simple paper-and-pencil, multiple choice exam. IQ tests often contain logical word problems or problems involving shapes and numbers, but such problems may not necessarily form an accurate picture of intelligence. For instance, a person with a very high IQ score may have no talent for music, while another person with an average IQ score may be musically gifted. A math whiz with with a high IQ might be terrible at creative writing, while a person who is poor in math and has an average IQ might be creatively gifted.
Another failing of IQ tests is the inability to evaluate characteristics that are important to achievement in the world that may not be directly related to IQ scores. For instance, the ability to interact well with others is an essential skill that many consider to be a form of intelligence, but this is not measured by IQ tests. Other important personal characteristics such as motivation to succeed, disposition and self-discipline are also not measured by IQ tests.
Perhaps the most important qualities in terms of employment opportunities are skills and knowledge. Skills and knowledge are learned through study and practice, but these qualities are not tested by IQ tests. Someone can have a genius level IQ but have little actual knowledge or skill, while another person with an average IQ may have spent years educating herself to obtain practical skills and knowledge.
Another problem with IQ tests is that results can vary considerably from one test administration to another. Even a difference of 5 points in the result of an IQ test could move someone from what is considered a low IQ to well within the average range. Another issue is that not all IQ tests are the same and difficulty may vary, meaning that a certain score on one test may not be equivalent to the same score on another.