MIT's Dspace@MIT has had 17,400 articles deposited and over 3.3 million downloads.
Here's an excerpt from the announcement:
A new milestone was reached in collecting articles under the MIT Faculty Open Access Policy : 43% of the articles published by faculty since they adopted their Policy are now being shared through the Open Access Articles Collection in Dspace@MIT.
As of the end of July 2015, downloads of the 17,400 articles deposited in relation to the Policy topped 3.3 million, with over 83,500 downloads during the month.