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 Biogen Idec (NASDAQ:BIIB) gained 3% today after J.P. Morgan upgraded the biotech giant from neutral to overweight.

So what: Along with the upgrade, analyst Geoff Meacham raised his price target to $375 (from $300), representing about 21% worth of upside to yesterday's close. So while momentum traders might be turned off by Biogen's price weakness in recent months, Meacham's call could reflect a sense on Wall Street that the company's growth prospects are becoming too cheap to pass up.

Now what: According to J.P. Morgan, Biogen's risk/reward trade-off is rather attractive at this point. "With respect to the MS franchise, we conducted a survey of 50 neurologists that points to upside for Tecfidera," noted Meacham. "Indeed, we have also fine tuned our MS estimates and are above consensus for Tecfidera, which is the key near-term commercial driver." With Biogen shares now up more than 60% from their 52-week low and sporting a P/E of 40, however, I'd hold out for a wider margin of safety before betting on that bullishness. 

Brian Pacampara has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.