How legally accurate are shows like "Law and Order"?

I’m on a “Law and Order: SVU” kick. The boy and I watch episodes together sometimes, and occasionally get into debates about the legality of things the DA/Detectives do, and about the way the show portrays the law.

I know all shows take some creative license, but in general, are these types of shows legally accurate? Is there a legal expert or fact-checker on the writing staff? Or does the show make up how the law works for entertainment?