At The Motley Fool, we poke plenty of fun at Wall Street analysts and their endless cycle of upgrades, downgrades, and "initiating coverage at neutral." While the pinstripe-and-wingtip crowd is entitled to its opinions, we've got some pretty sharp stock pickers down here on Main Street, too. (And we're not always impressed with how Wall Street does its job.)

Given this, perhaps we shouldn't be giving virtual ink to "news" of analyst upgrades and downgrades. And we wouldn't -- if that were all we were doing. Fortunately, in "This Just In," we don't simply tell you what the analysts said. We also show you whether they know what they're talking about.

Today, we're going to take a look at three high-profile ratings moves on Wall Street: A downgrade for ExxonMobil (NYSE: XOM) and a new sell rating for Polypore (NYSE: PPO) balanced by two buy ratings on hard drive maker Seagate (Nasdaq: STX). Let's dive right in.

Downwardly Mobil
Beginning with the bad news, shares of ExxonMobil are lagging the Dow's rally this morning, hurt by a downgrade to "hold" at analyst Argus. Exxon reported Q4 earnings yesterday, informing investors that production volumes were down, and profits up a mere 6.5% from last year's Q4.

Many analysts on Wall Street pointed to Exxon's $10 billion in capital spending for the quarter -- with little to show for it -- as reasons to worry about the stock. I see their point. Just a decade ago, it took Exxon an entire year to spend $10 billion on capex. Now it's got to do that each and every quarter? It would make sense if profits were rising proportionally, but they're not.

Now Argus is adding another worry: Pointing to Exxon's 2009 purchase of XTO, a deal that made the company America's biggest natural gas producer, Argus worries that the company's become top heavy in natural gas. In terms of energy equivalence (the amount of energy you can produce from burning one fuel or the other) natural gas is selling for just a fraction of the price of oil. Exxon may have doubled down on gas at exactly the wrong time.

Polypore: Full of holes?
Next up is Polypore -- and this one's going to surprise you. Yesterday, shares of the membranes maker (and Motley Fool Rule Breakers recommendation) fell off a cliff when key customer LG Chem announced it would begin competing with Polypore in the business of building battery separators for use in rechargeable batteries for electric cars.

As you may recall, both General Motors (NYSE: GM) and Ford (NYSE: F) have named LG Chem their preferred provider of car batteries for, respectively, the Chevy Volt and electric Ford Focus. It makes sense that LG Chem would want to keep as much profit from this business as possible in-house. It also helps GM and Ford, which may benefit from lower parts costs, as LG cuts out the middleman. But it's certainly not good news for Polypore, and it prompted a 26% sell-off in the stock yesterday. This morning, the analysts at Wunderlich responded to the news with a brand-new sell rating on Poly's stock -- only to see the stock bounce back 12% in Wednesday trading.

Did Wunderlich blow the rating? I don't think so. On the one hand, the best I can say about Polypore is that after today's spate of short-closings, the shares might be fairly priced at a 20% projected growth rate and a 20-times- earnings price tag. Considering that Polypore generates absolutely no free cash flow from its business, however, I have serious doubts about the quality of its earnings. When you consider further that Wunderlich ranks near the top 5% of analysts we track on CAPS, I wonder if Wunderlich might be onto something here.

See Seagate's stock price rise
I don't want to end this column on a down note, however, so let's wrap up with some good news for Seagate shareholders. Last night, Seagate reported a tripling of quarterly profit versus last year's fiscal Q2. Thanks to supply chain interruptions from last year's flooding in Thailand, the hard disk drive maker has been able to command premium prices for its product, leading to outstanding earnings of $1.28 per share.

This morning, the news prompted Stifel Nicolaus to upgrade Seagate shares to "buy," while Brean Murray, which already recommends the stock, boosted its price target 15% to $30 per share. Of the two, I find Stifel's upgrade particularly instructive, inasmuch as this analyst ranks among Wall Street's Best stock pickers, outperforming nearly 97% of the investors we track. I also think Stifel's right about Seagate.

Yesterday's profits report did yeoman's work in pushing down Seagate's P/E ratio to below 12. For a company that most analysts expect will grow earnings north of 30% per year over the next five years, this is a very attractive price. Even after today's 21% spike in share price, I think Seagate has room to run.

Whose advice should you take -- mine, or that of "professional" analysts like Argus and Wunderlich, Stifel and Brean Murray? Check out my track record on Motley Fool CAPS, and compare it to theirs. Decide for yourself whom to believe.

And if you're looking for more profitable investing ideas in the world of tech, read the Fool's new -- and free -- report on the industry: "The Next Trillion Dollar Revolution."