Unfortunately iMessage only works with other Apple devices. That is why Hangouts is nice, it allows multi platform communication. Whereas Apple wants to do its own thing.

I don’t know. Hangouts seems more efficient than Google Voice. The UI on Google Voice wasn’t that appealing, so now they have kinda killed GV and forced you to make calls through Hangouts. I personally enjoy Hangouts. It is prefect for communicating with teachers during school.