Obviously, the app is only useful if you’re already paying for WatchNow, but if that’s the case, having an additional screen to consume movies and TV shows is a welcome development. (Note that this is a quite separate app to the existing Quickflix app which lets you track physical DVD rentals.)
Realistically, you'll want an iPad for this; I can't think of too many scenarios where the iPhone/iPod touch version will be as useful, save for if a kid wants to watch something on their Touch or when you're in a hotel with free Wi-Fi.
On a related note, the app notes warn that it is "only available for streaming in Australia and New Zealand"; if you try and use it overseas on a Wi-Fi connection, sounds like you might get geo-blocked.
Quickflix is still working on an Xbox 360 WatchNow option which is due later this year.