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 Halliburton (NYSE:HAL) gained slightly this morning after Wells Fargo upgraded the oil-field services giant from market perform to outperform.

So what: Along with the upgrade, analyst Matthew Conlan planted a price target of $57 on the stock, representing about 14% worth of upside to Friday's close. While momentum traders might be turned off by the stock's slump in recent months, Conlan believes Halliburton is becoming too cheap to pass up. 

Now what: Wells lowered its 2013 earnings-per-share estimate for Halliburton from $3.15 to $3.11, but still likes the stock's risk/reward trade-off. "We think HAL is currently valued at an attractive (re)entry point and are upgrading the stock to Outperform from Market Perform (Reminder, we downgraded HAL to Market Perform on 9/9/13)," noted Wells. "Shorter-term, we are lowering our Q4 estimate to $0.89 from $0.93 to adjust for more difficult early winter weather in NA as well as a reduction of our Europe/Africa/CIS margin estimate." With Halliburton shares off more than 10% from their 52-week highs and trading at a forward P/E of 11, it's tough to disagree with Wells' long-term bullishness.