As well as being web-based and available on the mobile web, there's an Android app for the service and one for iPhone and iPad is in the pipeline – indeed, we hear it's already been submitted to the App Store for approval.
Here's what the mobile web version looks like on the iPad. Again it has the key components of Google+ as well as Notifications.
And here's what the Stream looks like - pretty standard.
However, the Android app is a little better in terms of design.
The Android app can instantly upload web-optimised photos to Google+ (called Instant Upload). You can uncheck this if you don't want it - presumably the iOS app will be able to do the same.
Here's the main screen of the Android app...
...where you might have noticed Huddles, basically instant messaging between mobiles. This is very easy to use, but don't get it confused with Hangout which, at the moment, isn't for mobile use.
And here's the Notifications display....
...and the Stream. It's all rather well designed, and it's easy to update your status as well as add media and share your location. A lot of time has been spent on this app and it'll be worth it. Mobile use will make or break Google+.