UPDATE: This issue is still present in Xcode 7.1 beta 2. Looks like this will be here a while.
I just updated to Xcode 7 GM and am in the process of generating ad-ho
Xcode generate .ipa file automatically on your desktop. It should be in the directory named from your app+date. If you cannot find it then try: CMD+space and type [name of archive].ipa and you should find it :) Hope it helps.
It looks like the latest version of Xcode (7.3) allows you to select the destination and folder name that contains the IPA file when generating ad-hoc releases.