Blogger Science Professor says that the question of whether to earn an MS before, or while, pursuing a PhD is one that she fields often. In general, she says, the question is "impossible … to answer, because the 'right' answer will vary." It depends on the goals and skills of the individual, the field, and the program's perceptions of the two degrees. "Some graduate programs view the MS as a consolation prize for failed PhDs; others require the MS as a useful 'weed out' step on the way to the PhD," she says. For careers in industry, government, and education, among others, Science Professor says that a master's degree is "certainly" useful.
In a comment to this post, Psyc Girl says that in her department, "when someone applies to our graduate program with an MA/MS I would say it is sometimes considered an unofficial black mark." However, in another comment, GMP says that in her discipline, a master's degree, she adds, can "bump one's employability and salary and is probably a better value for many industry-minded students than a PhD." Finally, Prof-like Substance views an MS as a trainee's "first real intro to what being a scientist is all about," he says. "For the MS students I have either directly or co-supervised, nearly all of them use it as a solid foundation for considering what they 'really' want to do and often feel the experience was instrumental in focusing them on their future goals, be it in a PhD program or elsewhere."