Valerie Spezi, et al. have published "Open-Access Mega-Journals: The Future of Scholarly Communication or Academic Dumping Ground? A Review" in the Journal of Documentation.
Here's an excerpt:
This paper represents the first comprehensive review of the mega-journal phenomenon, drawing not only on the published academic literature, but also grey, professional and informal sources. The paper advances a number of ways in which the role of OAMJs in the scholarly communication environment can be conceptualised.