Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: RSC Holdings (NYSE: RRR) shares climbed as much as 12% in intraday trading Friday after Oppenheimer upgraded the equipment rental specialist to outperform from perform.

So what: Citing a rental services market that seems poised for a rebound this year, Oppenheimer also offered a $15 price target on RSC Holdings' shares, representing about 40% worth of upside to yesterday's close. Not surprisingly, equipment rental rivals United Rentals (NYSE: URI) and H&E Equipment (Nasdaq: HEES) are also up handsomely on the forecast.

Now what: I'd be cautious about buying into RSC Holdings. While betting on a sector turnaround might pay off in the short term, the company's debt-to-capital ratio of 1.2 is just too dangerous for most long-term portfolios. And when you consider that these shares have now gained more than 50% over the past three months alone, the decision to stay away is that much easier.

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