Sounds promising. But not sure what this adds on top of Hangouts, which has a great group (and of course one to one) video calling feature. The only difference seems that Duo works with phone numbers, not Google Accounts.
Which is a huuuuuge difference. This is the same factor that made whatsapp so popular on the smartphone compared to all other other chat apps that were popular at the time (including google talk) but where each system was using a different account creation/login flow on that tiny screen, Whatsapp came along and had the brilliant insight/accident of simply supporting your phone's addressbook to find you and your friends. Even facebook chat couldn't keep up with them even though facebook app was installed pretty much on every internet connected phone at the time. Rest is history.