Very unresponsive -- both the app and the developer.
Very often the messages sent to me are not reflected on the app. I have to manually refresh the conversations to get updated.
The developer is equally unresponsive. I emailed them and told them of the problem. Someone replied and just asked me to reboot my Mac and my iPhone, which I have done many times. And when I told them their suggestion didn't work, they stopped responding. Rubbish app, rubbish company.