With proprietary solutions you often get the collective knowledge of the current and past users of that software or service. With open source software, at least for open source ISVs you often have a user base that's much larger relative to the size of the organization (than proprietary vendors that restrict distribution) and you get a great deal of leverage from the collective wisdom of a much large group. Plus it's often the case that you get very active forums, more user-contributed documentation, but also the transparency of the development process helps squash bugs more quickly.