Here's an excerpt:
PEER (Publishing and the Ecology of European Research), supported by the EC eContentplus programme, is investigating the effects of the large-scale, systematic depositing of authors' final peer reviewed manuscripts (so called Green Open Access or stage-two research output) on reader access, author visibility, and journal viability, as well as on the broader ecology of European research.
Peer-reviewed journals play a key role in scholarly communication and are essential for scientific progress and European competitiveness. The publishing and research communities share the view that increased access to the results of EU-funded research is necessary to maximise their use and impact. However, they hold different views on whether mandated deposit in open access repositories will achieve greater use and impact. There are also differences of opinion as to the most appropriate embargo periods. No consensus has been reached on a way forward so far.
The lack of consensus on these key issues stems from a lack of clear evidence of what impact the broad and systematic archiving of research outputs in open access repositories might be, but PEER aims to change this through building a substantial body of evidence, via the development of an "observatory" to monitor the effects of systematic archiving over time.