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.
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.