The LA Times really digs down into the pros and cons of DNA evidence in this feature article.
DNA is a huge help in forensic science. But the system creates strong incentives to push DNA evidence too far. The biggest problem in the court room is that the prosecution often misrepresents the odds of a DNA match, making it appear much more likely to be a match than it really is. As the article says:
Two national scientific committees, including the FBI's DNA advisory board, have recommended portraying the odds more conservatively. But interviews with expert witnesses and DNA analysts from crime labs across the country show that few if any have adopted that approach. The FBI lab, which oversees the nation's offender databases, has disregarded the recommendation of its own advisory board, bureau officials acknowledged.
Roger Koppl in this study for Reason shows the problems that come from having the forensic labs work for one side of justice system--the prosecution. He makes a great case and lays out how to make the forensic system more objective, and points out that the key to fixing the way DNA evidence is presented in court is ensuring that the defense can tap into forensic expertise.