Unless there's a setting I'm missing somewhere (which I've checked pretty thoroughly), this is how it works...
"Ok Google Now... Text <wife's name> Hey I should be there in about 15 minutes"
Wait a few seconds, it pulls up with my wife's picture with the text message underneath and the "send" option. It's not like some things (like a phone call) where there's a progress bar waiting for you that will eventually force the action... it waits until you hit send. If the text is off at all, I either start over completely (which is fine because you can use "Ok google now" again) or fiddle with it and edit the text manually.
It does the same thing for calendar appointments. And for phone calls, it defaults to the speaker being off - which, I would assume, if you're calling someone via voice command, you'd want the speaker on by default. But you have to hit "speaker" on your phone. The music takes a lot of "post voice command" work as well.
At this point (and hopefully this will change), it's no more intuitive than Voice Search, or hitting the "mic" next to the Google Search widget - they've simply added a way to "tap" that mic with your voice from anywhere and any state. That's very very cool in and of itself, but it's lack of intuitiveness after that initial command renders it somewhat useless IMHO. FWIW, general searches, navigation, etc. work fine - but again, identical to Google Now/Voice Search before.
Not to be trollish, but S-Voice on my S4 is 1000x more intuitive, even if it's not on its own core (for those that had the S3, S-voice is a lot better on the S4). I might have to wake S-voice with a button (unless I have it set to always on or in car mode), but from that point forward it's 100% voice. I can also customize wake commands and have a few different ones set. But S-voice works like this:
"Ok Galaxy (customizable)... text <wife's name> Hey I should be there in about 15 minutes"
"Ok, is this the message you want to send?" and shows you your message, but then prompts you via VOICE to continue... if I say "yes" or "send" or whatever, it'll send it... if I leave it in the S-voice app, she'll notify and read new texts back to me. If the text was wrong, I can say "no" or "edit" and she'll immediately ask for a new message. Again, complete voice interaction. Works the same for calendar appointments as well. Music isn't perfect on the S4 either - but if you haev music ON your device, you can use the S4's Music app and control it with "next, previous, pause, play, volume up, volume down" - and it works great as long as that music isn't too loud!
If they integrated Moto Assist more with Google Now it would be great... but MA is limited to a generic text response right now as well.
Holy cow that became long, sorry...