There is an article in the latest Library Resources & Technical Services issue, “Literature of Acquisitions in Review, 2008-9,” by Jeanne Harrell. Overall, it's an excellent snapshot of recent developments in library acquisitions and broad in its coverage. In the section “Approval Plans” she cites another article, “Collection Development and Outsourcing in Academic Health Sciences Libraries: A Survey of Current Practices” (Bulletin of the Medical Library Association 87, no. 2 (Apr. 1999): 178-86) that reported a “direct correlation between the size of a library's materials budget and its use of approval plans,” such that libraries with larger budgets are more likely to use approval plans. She mentions also that approval plans are a “time-saving tool for subject librarians with increasing demands on their time.” While these observations come directly from an article (which I haven't read yet) that looked at health sciences libraries, the results jibe with my experience in academic libraries with broader collecting practices, and which rely more heavily on monographs than I think health sciences libraries do.
Given the value of approval plans to academic libraries in particular, here I offer observations on factors affecting the success of approval plans in libraries.