A large percentage of the clients who come to me for career counseling wind up (or sometimes even begin with) contemplating a return to school as an option. Nationwide, the most popular (measured by enrollment) graduate programs are, #1 (by a large margin), education, followed by business and then by "health sciences" (medicine, psychology). Often a graduate degree is a necessity for work in a given field: obviously one needs an advanced degree to teach courses in a subject as a university professor, a law degree to become a lawyer or a medical degree to become a doctor, but a huge range of other positions require advanced degrees (for example, many positions at the World Bank require a Ph.D. in economics; a reasonably large number of higher government positions also require an advanced degree). Unsurprisingly, the number of jobs that will require advanced degrees is projected to increase at a faster rate than the number of total jobs over the next ten years.
Does that mean that enrolling in graduate school is a smart decision for mid-career professionals? The wisdom of the decision depends on several factors:
1) What is the primary motivation for returning to school? If you find yourself stalled in your career through the lack of a required degree or breadth/depth of knowledge in a field, pursuing an advanced degree may indeed be a smart move. If, however, you have been having difficulty finding employment with your current resume and network of connections, acquisition of a new degree is pretty unlikely to turn the tide. Academic knowledge is much less valued in the workplace than is practical eperience (and demonstrated accomplishments). You'd be better off investing in reconstructing your professional narrative (your "elevator speech," your resume), getting interview coaching, strengthening your network, pursuing volunteering opportunities, etc. By the way, a particularly bad reason to consider returning to school is that you don't know what other move to make (you'd be surprised at how many clients are motivated at least in part by this uncertainty).
2) What is the cost / benefit tradeoff? The first cost that comes to mind in cosidering pursuing an advanced degree is of course financial. A full-time two year graduate degree program at a decent school could cost $100,000. If you've been earning $75,000 and can reasonably expect an advanced degree to raise your income by 10%, it would take almost 14 years or the investment in your education to pay off. Fortunately, some of my cleints who work for particularly enlightened or progressive employers are able to have their organizations support their advanced studies with loans or subsidies.
Keep in mind that advanced degrees in certain fields confer very little financial advantage. An advanced "atmospheric science" (meterology) degree, for example, only adds 1% to post-graduate income. A Master in Fine Arts degree, 3%. Even a degree that might intuitively have great value, for example computer engineering, adds only 16% to one's average post-graduate income.
Other financial tradeoffs to consider: would pursuing a graduate degree prevent you from engaging in some paid part-time work, or investing time and effort in building your own business? Other significant costs to consider are those related to such difficult-to-quantify factors as self-esteem: I have worked with a number of clients who have dropped out of graduate programs either because they found the work too challenging, or were consistently depressed by comparing themselves to their much younger classmates. Then there are the preparation costs - unless you have a photoraphic memory or are a very adept test-taker, studying for the GREs or the GMATs is time consuming and often expensive (many clients hire tutors to help them and this can add up to a significant outlay).
3) Full-time or part-time? A large number of schools offer graduate programs that can be pursued on a nighttime or weekend basis, allowing for the continuation of income-producing employment. Beyond the standard advanced degree programs, many top (and not-so-top) graduate schools offer "executive" programs that confer what are essentially certifications; generally these are not all that valuable, although a Harvard, Wharton, or Stanford stint on your resume can certainly help, not to mention the high-level connections one can often make at such blue-chip graduate programs.
A final factor to consider is: how much do you like learning? To many, school is a chore, a burden. But if you are energized by learning, enrollment in a graduate program may be just the thing you need to launch, or relaunch, a successful career.