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.

For the last five years dating back from March of this year, Standard & Poor's MLP index returned 28 percent annually, compared with 22 percent for the 500-stock S&P index.

This performance has not gone unnoticed, and the number of MLP funds has grown in recent years.

The amount invested in MLP open-end and ETF funds increased from $2.7 billion at the end of 2010 to $32.9 billion at the end of last year, according to Morningstar.

There is no denying that MLPs have been a good bet for awhile now, said Evan Welch, chief investment officer for Antaeus Wealth Advisors in Boxborough, Mass. However, he fears that some individual investors may not understand that buying a peice of an MLP is different than buying the typical stock.

One difference is the investor is a limited partner, Welch noted (the general partners run the company). This has tax ramifications that the investor needs to be aware of, he said, whether the investor buys into an MLP stock or an MLP mutual fund. Depending on how the company or fund is structured, investors could be surprised by getting a K-1 tax form annually, and have to deal with capital gains issues, Welch said.

Others worry that with all the attention and controversy over corporations ducking taxes, that Congress may decide to eliminate the MLP corporate exemption.

Kelly said a change is always a possibility, but Tortoise keeps a close eye on Congress, and "we don't expect a change. Right now, it's going in the other direction,'' with the exemption expanded to include biofuel companies, she said.

Dan Pickering, a partner with Tudor, Pickering Holt & Co., Houston, believes Congress would be wary of fooling with an exemption that benefits, indirectly, so many senior citizens and retirees. These people vote, he noted.

Pickering likes the fundamentals of MLPs, so he is optimistic about their prospects going forward.

Kelly likes the outlook, too. She said that as of now, $125 billion is committed to be spent on new pipeline construction in North America through 2016.

Tortoise keeps a close watch on the industry, she said, with 55 employees dedicated to watching everything "from the bottom up," and proprietary methods of tracking performance and prospects.

These people watch not only the so-called "midstream" pipeline and storage tank companies, but also the "upstream" producers and "downstream" distributors, Kelly said.

Tortoise, which has $19.2 billion under management, chose that name to symbolize their belief in the "slow and steady wins the race" virtues of that creature, and its virtues of tenacity, patience and resilience, according to the company Web site.

The Web site even has a video of a tortoise exhibiting those qualities in action. It is not as compelling as the classic "lion prowling Wall Street" black-and-white commercial that helped make the Dreyfus Fund such a success in the 1960s. But it is a distinctive piece of footage.