The second quarter generally ended with a bang for the likes of ExxonMobil (NYSE:XOM), ConocoPhillips (NYSE:COP), and Chevron (NYSE:CVX) -- each of which delivered solid net-income and earnings-per-share figures -- thanks, in large part, to the current sky's-the-limit price of crude.

Despite the strong numbers, though, you must ask yourself whether now is the time to take the energy stock plunge. Should current investors contemplate taking their profits and going home, or at least head in the direction of a less frothy area of the market?

Good questions both, and not surprisingly, there are good answers on either side. Far be it from me to make a market call, of course. And with that in mind, here are some points worth contemplating as you mull the cases for and against investing in energy stocks right now.

Just do it
Despite their impressive run-up of late, the aforementioned dynamic trio still looks cheap in terms of relative valuations. On a trailing 12-month basis, for example, each of 'em currently trades at a price-to-earnings discount relative to the broader market, as well as to industry peers and their own five-year averages.

Moreover, particularly in the case of ExxonMobil, the market doesn't yet seem to have priced in what we'll call a "size matters" premium. True, like its competitors, ExxonMobil's fat bottom line owes substantially these days to a favorable supply-demand curve and to the soaring prices that curve has sloped into.

But also true, the firm boasts the kind of operational efficiencies that only the large, effectively managed companies can muster. And make no mistake: Despite the upcoming departure of CEO Lee Raymond, ExxonMobil is an especially well-managed company, one with a reputation for keeping its "upstream" operations (i.e., exploration and production) firing on all cylinders and its "downstream" functions (i.e., refining and distribution) typically running with a minimum of slack.

Chevron hasn't been quite as strong in the latter regard, particularly not during this year's second quarter. Over that stretch of time, maintenance of certain of its facilities contributed to a downstream profitability downturn compared with the same period last year.

Still, Chevron has been aggressive when it comes to capital spending -- and also when it comes to looking abroad for new fields to tap. Indeed, the lion's share of the billions of dollars the company doled out on exploration projects during the first half of 2005 went to international efforts.

Given the maturity of its domestic oil patch, this penchant for global exploration bodes well for the company's fiscal future. The same is true of Chevron's pending acquisition of Unocal (NYSE:UCL), which, once all the dust settles, will likely further bolster the company's prospects in an industry that rewards girth.

Don't just do it
All of which brings us to ConocoPhillips -- a company that comparatively offers a good illustration of why now might not be a such a good time to load up on energy shares.

There's a reason that even the best of these companies trades relatively cheaply, after all. Namely, they're commodity businesses and are therefore subject to the ebbing and flowing (so to speak) of their particular commodity: crude. Among other things, it's the potential for volatility at the core of their businesses that helps keep a lid on these firms' price multiples.

With that as a backdrop, given its size - ConocoPhillips' market cap is smaller than both ExxonMobil and Chevron - this more modest energy titan is more susceptible to downward crude oil price swings.

But hang on a minute, COP fans. Before penning those scathing emails, hear me out first.

There's no doubt about it: ConocoPhillips has delivered for shareholders over the long haul and is currently going gangbusters on a year-to-date basis: Through yesterday's market close, the company's stock is up nearly 54% thus far in 2005. Moreover, while ExxonMobil and Chevron represent more compelling investment opportunities just now -- in part because their stock prices haven't spiked so sharply of late - ultimately, we're talking about three peas in an industry pod here. At least when it comes to the impact of oil prices.

In that respect, ConocoPhillips could prove to be a proverbial canary in a coal mine: Its stock price might fall first if and when the oil price bubble finally bursts, but it's likely that the firm's bigger brothers' shares wouldn't be all that far behind.

What's more, beyond the commodity nature of the energy business, there are more forward-looking concerns to contend with: Someday, we're surely going to shake our addiction to fossil fuel, right?

If you think that sounds a bit Panglossian, perhaps you'll at least concede that along the way to the best of all possible energy worlds, hybrid vehicles such as those made by the likes of Honda (NYSE:HMC) and Toyota Motor (NYSE:TM) are sure to gain in popularity. Heck, there's even a tax credit for 'em.

The Foolish bottom line
That said, it will no doubt be a long, long while before we're fossil fuel-free. In the interim, the energy sector's major players -- which also include the likes of BP (NYSE:BP) and Total SA -- still pack plenty of appeal for long-term investors with realistic expectations.

Fat earnings powered by oil prices in excess of $60 a barrel, that is, aren't likely to last -- at least, not if history is any guide. But by focusing closely on each company's fundamentals and by zeroing in on those companies whose operating efficiencies set them apart from the pack, Foolish investors intent on tiptoeing through the oil patch may tap superior, long-term returns.

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Shannon Zimmerman is the lead analyst for Champion Funds and doesn't own any of the securities mentioned. Total SA is a Motley Fool Income Investor recommendation. The Fool has a strict disclosure policy, which you can read all about by clicking righthere.