Apple’s posted this graphic (h/t Robert Davey) in a public page of its developer portal:

iOS 8 was released on September 17th and the chart was last updated on Sunday, Sept 21st, so the data represents the first 4 days of release.

This compares favorably with ad network Chitika’s report of tepid iOS 8 adoption posted within a day of the release, showing a 24 hour adoption number of 7.3% for iOS 8 as opposed to 18.2% for the first 24 hours of iOS 7.