While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of PepsiCo (NASDAQ:PEP) gained slightly this morning after Jefferies initiated coverage on the soft drink and snack gorilla with a buy rating.

So what: Along with the bullish call, analyst Kevin Grundy planted a price target of $102 on the stock, representing about 15% worth of upside to yesterday's close. So while contrarian traders might be turned off by PepsiCo's steady price strength in recent months, Grundy's call could reflect a sense on Wall Street that its growth prospects still aren't fully baked into the valuation.

Now what: According to Jefferies, PepsiCo's short- and long-term risk/reward trade-off is rather attractive at this point. "PepsiCo's global snacks franchise is the gem in the portfolio, and will continue to drive a disproportionate share of growth and value for investors," said Grundy. "Nascent signs of improvement in PEP's NA beverage business (NAB) are encouraging and should improve sentiment for what has long been an underperforming asset. Activist involvement -- if nothing else -- should elevate management's game and drive better execution." When you couple that upbeat outlook with PepsiCo's still-juicy dividend yield of 3%, it's tough to disagree with Jefferies' bullishness.