The Audience Report provides a current view of your audience by the number of devices opted-in and out- of notifications for iOS and Android. It also includes your total opted-in uninstall count. The data is updated daily and you can view previous snapshots (back to Jan 1st, 2014). The report is downloadable in CSV format making it easy to download multiple snapshots for comparison.
This is now the Devices report.
For more details on this new report, see: Reports Menu Guide: Report Types: Devices.