So, I've been using Vivaldi every day for months. It's not my workhorse (yet).
Why Vivaldi? Well, I was an Opera user going way, way back before Chrome, before Firefox, before Internet Explorer even existed I think. I started using Opera when Netscape was all the rage.
Unfortunately, Opera decided to jump from version 12.17 to 15. Versions 15 and up are built on Chromium, the same engine behind Chrome. Sadly, Opera did not make its new version like Opera 12. It dropped all sorts of really fantastic features I was using all the time (and still do, as I still use Opera 12 more than any other browser, though I have to do more and more workarounds to do it because websites are no longer supporting it). It simply still does things that others don't, including those others when loaded with extensions. Plus, my macros are written for Opera 12.
Anyway, the people behind Vivaldi are from the Old-School Opera team, and they have created a new browser based upon the old "Opera philosophy," which you can read about in the linked article.
I support their efforts and hope more and more others will too so Vivaldi's developers may have the user-base backing to bring more and more great features back into the browser space and to develop new features too, all that will be standard to the most up-to-date website needs.