According to "Requirements for a Competent In-House Opinion," by Vineet Kohlia, competent and reasonably relied upon opinion letter can be of great value to defeat a charge of willful infringement:
[I]t appears that the test for competency of an in-house opinion letter is that, the opinion demonstrates (a) the authors expertise and experience, (b) the absence of a material, financial or other interest, and (c) the existence of other indicia of reliability and objectivity. . . . The competency of the opinion letter depends not only on its overall thoroughness but also on the competency of its author. In addition to being competent, the evidence must also establish the infringerâ€™s good faith reliance on the opinion letter to justify a good faith belief that a court of competent jurisdiction will likely find the adverse patent invalid, not infringed or unenforceable. That the ultimate conclusion of law reached by the author of the opinion are found to be incorrect, matters not, as long as the opinion letter is objective and reliable."