Master limited partnerships -- companies that provide pipelines and other infratructure for energy producers -- have enjoyed good weather and smooth sailing in recent years.

However, some observers see storm clouds on the horizon.

MLPs have prospered for several reasons. They have an exemption from corporate taxes, so they can pay out more to investors in dividends. Energy production has expanded in North America, primarily because of more production from shale deposits. MLPs' popularity with investors has grown because of low interest rates. They are viewed as a relatively low-risk way to get a higher yield than one gets with government bonds.

Nonetheless, not everyone is completely sold.

"At the top, everybody's a believer," Timothy Gramatovich, chief investment officer for Santa Barbara, Calif.-based Pentus Asset Management,, told Bloomberg Businessweek in March. He sees MLPs as "the next great investment debacle."

Efforts to reach Gramatovich for this article were unsuccessful.

Few are as gloomy as he is about MLPs. But even those who are bullish on the space are not unreservedly so. They say, essentially, "let the buyer beware." And they suggest that those who want to jump on the bandwagon should either know what they are buying or buy from those who do know, such as the managers of actively managed MLP mutual funds.

"Not all MLPs are created equal," said Michelle Kelly, a managing director of Tortoise Capital Advisors LLC, a Leawood, Kansas-based firm that specializes in MLP investments.

Tortoise, which was started in 2002, created the first listed MLP mutual fund in 2004. It offers several closed- and open-end funds. The company also manages MLP portfolios for pension funds, endowments and other institutional investors.

Its Tortoise Pipeline Institutional Fund, or TORIX, had a three-year return of 26.24 percent as of Sept. 19, according to Morningstar, and a one-year return of 33.65 percent, putting it near the top of the MLP funds during those periods. The fund was created in May 2011.