I also just came from Windows Phone, and I find it amazing that this "feature" isn't baked into Android. My first Windows Phone 7, back in 2010, could do voice controlled text reading/replying when on headset or Bluetooth... It got way better in Windows Phone 8 with Cortana's debut. With Windows 10 Mobile, however, I found that the feature actually go worse. Not sure how that happened, but for my car (a Ford with Sync), the voice texting became near useless, a real hit or miss proposition.
Now I have a Nexus 6p. I found that S Voice that some of you are talking about here is an exclusive Samsung thing. I've been searching through the Play store and haven't seen anything like what we could do with Windows Phone. Found some that will read the messages, but I'm not sure about replying. Of course, there are some many different apps out there, I'm probably just not finding it.
Since I'm probably only one of two other guys here in this forum who have ever even seen a Windows Phone before <sarcastic tone>, this is how it worked on Windows:
First, you turn on the voice assist feature when connected to a BT headset or other device.
When connected to your headset (or car) an incoming text will trigger Cortana to tell you "you have a message from xxxx do you want me to read it," etc.
You say "read it," and Cortana will read the message to you. When done, she asked if you want to reply.
You speak back your reply, and Cortana will read it back to you and ask you if you want to "try it again, add more, or send it". If you're good to go, you can just say "send it."
The nice thing is that this feature isn't an "app" as it's just part of Windows Phone and you didn't have to start or launch anything (other than to turn it on in the first place). It wasn't perfect, like you wouldn't want to try this driving on the highway with your windows down, but for the most part it was a great feature.
Does anyone know of an app that will do this?