I use it as it's the only cross OS app that supports Wifi to phone network switching in a call (others end the call when you lose connection), but I really don't like it. It's exactly as it was when they published it. iOS supports call screens for other apps since a while now (which whatsapp uses), Duo calls still only come as a notification. And they also sometimes fail if the other person hasn't used the app for a few days.
I really liked the concept of the app, and the quality is certainly at least on par with other apps. But I have the strong feeling that it won't be around for long.
Let's hope Whatsapp supports network switching soon so that I can get rid of Duo.