In my experience, IQ tests are a good indication of your ability to take IQ tests and very little else. Of course, you can differentiate an absolute dunce from someone who's not, but nothing more subtle than that.
Personnel selection research provides much evidence that intelligence (g) is an important predictor of performance in training and on the job, especially in higher level work. This article provides evidence that g has pervasive utility in work settings because it is essentially the ability to deal with cognitive complexity, in particular, with complex information processing. The more complex a work task, the greater the advantages that higher g confers in performing it well. Everyday tasks, like job duties, also differ in their level of complexity. The importance of intelligence therefore differs systematically across different arenas of social life as well as economic endeavor. Data from the National Adult Literacy Survey are used to show how higher levels of cognitive ability systematically improve individuals’ odds of dealing successfully with the ordinary demands of modem life (such as banking, using maps and transportation schedules, reading and understanding forms, interpreting news articles). These and other data are summarized to illustrate how the advantages of higher g, even when they are small, cumulate to affect the overall life chances of individuals at different ranges of the IQ bell curve. The article concludes by suggesting ways to reduce the risks for low-IQ individuals of being left behind by an increasingly complex postindustrial economy.
Employees will test the limits with lawsuits. I know at least one large state lost a class action lawsuit due to racial bias on civil service exams.
The problem with comparing IQ results at hire to employment success is that employment outcome is difficult to define over time. You're also unlikely to get statistically relevant data without focusing on large organizations with standardized HR processes. Most of the research is based on supervisory evaluations, which are not the most reliable indicators of anything for a variety of reasons.
The other thing I find amusing is that business folk who talk about this miss the fact that there are large workforces in the US that either have or do use standardized testing like this to hire and promote. Those are government bureaucracies, which function relatively well, but are hardly a model that most folks advocating this would aspire towards.