The strange thing is that even though the Hangouts app effectively takes over all of the text, voice, and video chat functions from Google+, the Google+ app still has a Messenger tab on its main navigation bar. At this point, we can assume that the Messenger tab will eventually go away, but for now, it somewhat awkwardly points you to the Hangouts app.
So why did Google break the Hangouts functionality out of Google+ anyway? I think it's partly for the convenience of the one-tap access to the app, but it's also to create a central app for all communications., community manager for Google+ Hangouts & Chat, SMS will soon be part of Hangouts as well. There is one distinct advantage right now for iOS users, however: you can make video calls across platforms -- something FaceTime cannot do.
As for performance, Hangouts had some minor delays, which are to be expected, but it worked fairly well on both Wi-Fi and 4G in my testing.
Overall, Google Hangouts does what it's supposed to do: it makes video calls and lets you message your friends. For iOS users, it has an advantage over Apple's FaceTime in that it can make video calls across platforms. So, if you want a slightly quicker way to access hangouts, or just don't want to get involved with the whole Google+ app, maybe Hangouts is a perfect fit.