While reading some articles on the Mars rovers, I saw this pair of pictures showing the Opportunity rover's solar panels first deployed in January 2004 and today − its solar panels are now very dirty (Source). There was a time when NASA would talk about cleaning events -- winds that would clear the panels. I do not hear much about cleaning events anymore.
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The dust has produced a remarkable reduction in daily energy output (Source).
As of Wednesday, Jan. 15, 2014, or Sol 3547, the solar array energy production on the rover is 353 watt-hours, compared to 900 watt-hours after landing.
Even though the daily energy output is less than 40% of its original value, the rover is continuing to do good work.
The Curiosity rover uses two nuclear batteries and it does not have the same sensitivity to dust as the Opportunity and Spirit rovers.