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

What: Shares of Shutterfly, (SFLY) gained about 1% on Friday after Goldman Sachs upgraded the online photo service from neutral to buy.

So what: Along with the upgrade, analyst Heath Terry boosted his price target to $52 (from $47), about 27% higher than yesterday's close. So while momentum traders might be turned off by Shutterfly's sharp pullback over the past month, Terry's call could reflect a growing sense on Wall Street that its prospects are becoming too cheap to pass up.

Now what: According to Goldman, Shutterfly's risk/reward trade-off is rather attractive at this point. "[T]he company continues to post at least mid-teens organic growth, leverages a three-year technology and fulfillment driven investment cycle into more sustainable growth, and potentially benefits from the development of a higher margin, less seasonal, more future-proof, cloud product in ThisLife," said Terry. "While consumer transition to digital cards, invitations, and photos remains a long term risk, ThisLife could represent compelling upside at the current valuation." Given how strong those digital headwinds are, however, conservative Fools would probably do well to just completely stay away.