David Nicholas et al. have published "Have Digital Repositories Come of Age? The Views of Library Directors" in Webology.
Here's an excerpt:
This survey of approximately 150 repositories assessed the achievements, impact, and success of digital repositories. Results show that while the size and use of repositories has been relatively modest, almost half of all institutions either have, or are planning, a repository mandate requiring deposit and small gains have been made in raising the profile of the library within the institution. Repositories, then, have made a good deal of progress, but they have not quite come of age.