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 low-fare airline Spirit Airlines (NASDAQ:SAVE) closed up 14.8% on Friday after posting strong September traffic and receiving an upgrade from Citigroup, from neutral to buy.

So what: Along with the upgrade, analyst Stephen Trent boosted his price target to $42 (from $38), representing about 7% worth of upside to where the shares sit now. While value investors might be turned off by the stock's hot run in 2013, Trent believes that there's room to fly given Spirit's strong September traffic growth of 29% and upbeat Q3 revenue guidance.

Now what: Citigroup doesn't expect Spirit's operating momentum to slow anytime soon. "We are upgrading Spirit Airlines from Neutral to Buy, on the back of very strong September traffic stats, including 3Q13 [revenue per available seat mile] growth guidance of 8% to 9% y-o-y," noted Citigroup. "Plugging this stronger guidance into our model boosts our estimates. This, along with the shares' ca. 1% decline since late July as the group has rallied, present a compelling backdrop for a Buy rating on the shares." With the stock now up 150% over its 52-week lows and trading at a 20-plus P/E, however, I'd wait for a much wider margin of safety before jumping into Spirit and, in turn, the notoriously intense airline industry. 

Fool contributor 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.