Looks very promising. It's what should have happened with opera, add customization to a mainstream rendering engine. It may harm performance to have all features and UI in javascript, but it also bring the ability to affix opera, erm Vivaldi, to any rendering engine. I think it is funny that a browser created only a few months ago is already better than new opera.
Their website is horrible though, and doesn't render well in presto. Looks like a designer ate up all the fad mobile/metro styles and pooped them out onto a web server.