NEW YORK (FORTUNE) - For millions of Americans, the first big financial decision in life is whether to take on a student loan. Student loans are debt, of course, but they represent something different than credit card debt or a car loan: They are part of a quest for a better future.

And they can have lifelong consequences, both good and bad, because for the unwise or the plain unlucky, a student loan can become an inescapable burden. It can almost never be expunged in bankruptcy, and the Supreme Court just ruled that even Social Security income can be garnisheed to pay for defaulted student debt.

The giant of the student loan industry is the Student Loan Marketing Association, better known by its friendly-sounding nickname, Sallie Mae. Many people think that Sallie Mae, like Fannie Mae and Freddie Mac, is sponsored by the U.S. government. And until recently it was. But at the end of 2004, Sallie became an independent, publicly traded company, completing a process begun in 1996.

It is now radically different than it was even five years ago -- an aggressive, highly profitable lender and a stock market superstar. Since 1995 its stock has returned over 1,900 percent, trouncing the S&P 500's 228 percent gain. Today Sallie's stock sells for 22 times earnings and almost ten times tangible book value, "an almost unheard-of valuation for a financial institution," as a Criterion Research report noted.

Sallie's dividend has risen at an average annual clip of 18 percent over the past ten years. And thanks to hefty helpings of stock options, Sallie's top executives have earned fortunes. From 1999 to 2004, just-retired CEO Al Lord -- now the lead investor in a group trying to purchase the Washington Nationals -- received total compensation of $225 million. New CEO Thomas "Tim" Fitzpatrick made $145 million over the same period.

To produce those sorts of numbers, a company usually has to be obsessed with the bottom line, and Sallie is certainly that (a big chunk of its executives' bonuses is based on Sallie's profits). As good as that may be for shareholders, a growing number of critics contend that those profits are coming at the expense of Sallie's other constituents: students and taxpayers.

"Sallie advocates policies we believe are frequently contrary to the interest of students," says Luke Swarthout, a higher-education advisor to the U.S. Public Interest Research Groups. He charges that Sallie used its political clout to shape new legislation that will increase the cost of student loans.

Ira Rheingold, executive director of the National Association of Consumer Advocates, decries Sallie's growing presence in the ugly business of collecting on defaulted debt. Pennsylvania state representative Doug Reichley alleges that Sallie is engaging in "predatory lending."

Indeed, Sallie uses high interest rates and fees to charge students as much as 28 percent annual interest on loans. As a result, some have seen their school-loan debt balloon into six-figure delinquencies that they can't hope to pay when the collection agency (which nowadays may be owned by Sallie) comes calling.

"Sallie Mae's practices are in the best interests of students, schools, and taxpayers," a spokesman says. "We were created 34 years ago to provide access to higher education for Americans, and that's still the business we're in. No one wins if a student borrower is unable to repay his or her loans."

You could also argue that student loans are simply a business, like computers or laundry detergent -- or maybe health care -- and that Sallie's sole responsibility is to deliver results for its shareholders. But lately there are even some doubts about Sallie's ability to do that, given the increased risks it is taking to sustain its high profits.

Two Wall Street analysts actually rate Sallie's stock underperform. Many investors buy the stock because they still see it as a safe, government-backed company. They may be in for a surprise.

To read the complete story on Fortune.com, click here.