To me, the whole idea of using voice search when I can just as easily type the exact query I want isn't appealing, but perhaps I should start changing my perception. Google's voice recognition algorithms are getting better and smarter, and one of the areas where you can notice this improvement is YouTube. Even more appealing now is the fact that YouTube immediately starts playing a video when you do a voice search.
To try this, open the search page on the YouTube app on your phone then tap the microphone icon on the top right. Start by saying "play" and then ask for any video, including one whose specific title you know or something more elaborate like the new song by an artist, the latest Avengers trailer, the Eurovision 2019 winner, or the trailer of the latest movie by Julia Roberts. The smarts behind this appear to be the same as Assistant's as YouTube is able to recognize instantly what you're talking about. And instead of just giving you a page of results, it starts playing the video immediately, without you having to select or tap to play.
Left: Previous voice search on YouTube. Right: New voice search.
When comparing with the old UI, you'll also notice that the new voice search overlay takes the entire screen and has a red microphone instead of a blue one. This new behavior is replicable on all our devices, so it does seem to have widely rolled out to users. It does appear to be new too — our tipster says it showed up for him on version 14.20.54 (APK Mirror) and we've just seen a tooltip encouraging us to search in new ways — but it's likely been in testing for some users for a few weeks if not more.