David Nicholas has self-archived "Digital Repositories Ten Years On: What Do Scientific Researchers Think of Them and How Do They Use Them?" at the CIBER Research Ltd.'s website
Here's an excerpt:
Digital repositories have been with us for more than a decade, and despite the considerable media and conference attention they engender, we know very little about their use by academics. This paper sets out to address this by reporting on how well they are used, what they are used for, what researchers' think of them, and where they thought they were going. Nearly 1,700 scientific researchers, mostly physical scientists, responded to an international survey of digital repositories, making it the largest survey of its kind. High deposit rates were found and mandates appear to be working, especially with younger researchers. Repositories have made significant inroads in terms of impact and use despite, in the case of institutional repositories, the very limited resources deployed. Subject repositories, like arXiv and PubMed Central, have certainly come of age but institutional repositories probably have not come of age yet although there are drivers in place which, in theory anyway, are moving them towards early adulthood.