I've been using Alexa, and Siri for awhile now. I gave Google Home a try for a few months, but dumped it (not enough integrations with IoT devices). The promise of voice landed with the dead thud I thought it would. That said, there have been a few use cases where I've found it immensely valuable.
Being able to control parts (automated blinds/shades, thermostat, lights) of my house via voice commands has been great. Alexa has the most integrations and home IoT devices are so brutally splintered, that in order to effectively play the field, the only option is Alexa. I'm sure there will be consolidation here, and Apple will eventually win that battle (though HomePod might not survive long enough for it to matter). I've been shocked at how literal the voice system is however. It's been challenging building carefully constructed strings of commands in my head and spewing them verbally into the air in order to get Alexa to do the right thing ("Alexa, turn off the 3rd floor fan."). I'm a bit surprised Alexa hasn't figured out more natural/ambient context. If she could figure this out, it'd be really helpful. Having to remember the command syntax for infrequently used commands basically means I can't ever commit the commands to memory, so in turn, I don't use them. With graphical UIs, I always have visual context to guide my memory and actions.
Talking To Me
I also use Alexa for morning news briefings. Periodically I ask her to read the "Sleep With Me" podcast when I'm having trouble falling asleep. I sometimes ask Siri to "read me my new messages" in the morning (I leave my phone downstairs each night so it's not in my bedroom) in my bedroom, or while I'm making coffee.
I use Siri for iMessage'ing and Reminders constantly. I also use OSX dictation periodically. Being free from my phone to message ppl (at home, or in the car via CarPlay) has been awesome! There are plenty of times, mostly while in the car, however when Siri gets it wrong, and that's annoying.
The usefulness of voice however is excruciatingly narrow. It is only usable when you are alone. You can't use it unless you're in a noise/distraction free environment. How often are you in a noise/distraction free environment? That's what I thought… basically never.