Like the Android update, this update brings one super nice feature to the iOS app: the ability to start a Hangout. Until now, Hangouts could be joined from the mobile app, but not created. Hangouts can only be started through Messenger, meaning you can't start a public Hangout without targeting one of your contacts for the initial startup. Of course, you can add users once you've created the Hangout, or even make the Hangout public.
The update also brings a new UI to the Photos section of the app, which is nice, and of course the obligatory bug fixes.
If you don't yet have the Google+ app, you can grab it for free from the iTunes App Store.