According to the Innocence project, there have been 289 post-conviction DNA exonerations in the US.  Other reports give different numbers - I don't know why there is a difference.  The link gives statistics that I find mind boggling.

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Death Penalty Information Center states 140 death penalty exonerations.

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In Texas, 41 exonerations in 9 years. 

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I suppose with more use of DNA, there will be fewer wrongful convictions.  In the last link, the Harpers article concludes "Some prosecutors argue that the reputation of the criminal justice system and our interest in keeping costs down require a policy that avoids looking back. They say that once a defendant has had his pass through the system, flawed though it may be, he has gotten all the law promises. But the integrity of the criminal justice system depends first on its ability to dispense justice, and that must include a recognition that prosecutors, judges, and juries make mistakes. By exposing their past injustices, courts and prosecutors in Dallas are revealing their own reinvigorated commitment to do justice and are converting a tarnished record into a beacon for the rest of the country.