Vivaldi is not open source so that's an immediate "no" from me. I'm fine with using closed source tools from time to time, but a web browser is just too important and intertwined in my life to trust a closed source offering.

In a perfect world we'd have nothing but fully open source software funded by support contracts and commercial interest in offering superior software, but the world isn't perfect and so we have to compromise sometimes. But the browser is one place I draw the line.