While at ACRL, I met a Librarian at Wellesley College by the name of Deborah Lenares. She was in the midst of evaluating ebook aggregator platforms and shared with me her comparative spreadsheet. It’s quite thorough and I thought it could be a great community project to work on it together. So, Deborah has posted this spreadsheet to google docs and it is open and available for editing by anyone.
Document is available here: https://spreadsheets.google.com/spreadsheet/ccc?key=0AtHuZMbGK8S1dGpsZHdhYmZsLUhqbW50STZlcTZPT3c&hl=en_US#gid=0
The University of California – Irvine also did a comparison spreadsheet. It is available for download here: http://www.libraries.wright.edu/noshelfrequired/2010/01/06/checklist-for-evaluating-patron-driven-business-models/
Finally, the Joint Information Systems Committee (JISC) in the United Kingdom maintains an academic database assessment tool for e-books. Users select vendors and the comparison data is displayed based on user choice. It is available here: http://www.jisc-adat.com/adat/adat_ebooks.pl