Nobody likes being late to a good party, but who's to say when the party's over?

If you're tempted to recoup the costs of your $40 fill-ups by investing in energy stocks, the status of the party weighs heavily on your mind. Is the sector's high-octane run about to end or is there still money to be made?

There's plenty of reasons to be tempted to party hardy. So far this year, three Standard & Poor's 500 indexes -- Energy, Integrated Oil, and Oil & Gas Equipment & Services -- have generated returns of 40, 32 and 39 percent respectively. By comparison, the S&P 500 returned less than 2 percent.

The energy indexes also handily outperformed the broad market over the one-, three-, and five-year periods ending Aug. 31.

The numbers bring to mind the boilerplate language investors ignore when betting on the hottest mutual funds: "Past performance does not guarantee future results."

The uneasy feeling that what goes up must come down, as the bursting of the tech bubble confirmed, leaves some investors wary of buying at this stage of the game. Last week, Morningstar cautioned investors to give energy stocks the cold shoulder, saying it expects oil to return to $35 a barrel by 2008.

"High prices like those today ... have always resulted in a market response that eventually leads to a significant decline in price," analyst Jonathan Schrader wrote in an article posted on the investment research firm's Web site.

However, it's not hard to find analysts eager to crash the party, especially on price dips. Colin Symons, chief investment officer of Symons Capital Management in Castle Shannon, is buying on dips, when energy stocks retreat 5 to 15 percent from their recent highs.

Earnings "estimates are generally assuming $40-45 oil, which is demonstrably wrong," he says. "We have been, and continue to be, fairly heavily overweight in energy stocks."

Symons likes oil service and equipment companies, a sector also favored by Charlie Gaffney, lead energy analyst for Eaton Vance.

Gaffney says oil service stocks are trading at 12 or 13 times the cash-per-share their operations are expected to generate over the next year. That's shy of the sector's historic peak cash flow multiple of 16. Shares of oil rig companies are trading at 10 or 11 times forward cash flow vs. their historic peak of 16 or 17, he says.

All of that provides fuel for a continued run up. "On a pure valuation basis, the stocks are not screamingly expensive," Gaffney says. "I'm still bullish on the sector."

While he won't give a specific oil price forecast, Gaffney doesn't expect prices to tumble. His best guess: $40 to $60 a barrel for the next two to five years.

"I think we're in a higher-priced environment for a longer period of time," Gaffney says.

Until recently, energy stocks were trading at a discount to their historic price/earnings ratio, says Charles Schwab sector analyst Brad Sorensen. The ratio reflects how much investors are willing to pay for $1 per share in earnings.

"Following the recent run, they've come pretty much in line with historical averages," Sorensen says. "We're certainly long-term bullish on energy. On a pullback, we'll be looking to going to a longer-term overweighting of the sector."

Donald Belt, chief investment officer for Hefren-Tillotson, isn't so sure.

While timing the top of the energy market is nearly impossible, "we have grown increasingly more concerned with the recent run-up in share prices and believe a cyclical correction is highly likely," Belt says. "A better opportunity to invest in the energy sector will lie down the road."

That's advice many dot.com investors wish they would have heeded in 2000, when they watched their paper profits go up in smoke.

During their bull run, tech stocks advanced at a more furious pace than energy stocks have to date. In 1999, the Nasdaq index produced returns of 86 percent. By comparison, the S&P 500 Energy index generated a 52 percent return in the 52 weeks ended Aug. 31.

Analysts note that most energy companies have proven business models and are consistently profitable. That couldn't be said of many of the dot.bombs. When their bubble burst and there were no earnings to provide support, tech investors lost a bundle.

Analysts expect fate will be kinder to energy investors when the cycle turns. "Energy may give some back, but they're not going to give the whole thing back," says Greg Melvin, chief investment officer for C.S. McKee, Downtown.

Sorensen believes memories of the tech bubble will temper investor behavior this time around.

"People, to some extent, learned from some of the mistakes they made during that bubble," he says. "That will limit the excess of speculation."

Investors with short-term horizons should be more worried about placing bets at the top of the energy market. Short-term investors who took that approach when oil prices peaked in 1980 saved their skin.

But some long-term investors who bought at the "top" of that cycle have done all right, too. If you had purchased shares of Exxon, now Exxon Mobil, in late 1980 and held on through all the cycles since then, you would have been amply rewarded.

Excluding dividends, 25-year Exxon shareholders earned annualized returns greater than 10 percent, beating the 9 percent annualized returns of the S&P 500.

Those numbers indicate there's always a party. You just have to know how to behave.

Trouble is, when money's involved, many investors aren't on their best behavior.