I installed both the Windows and OS X version, and it looks pretty good (aside from having that disgusting "page flattened" look of every fad-chasing program ever since Windows Metro and OS X Yosemite). However, I tried to search for a specific word in text, and the prediction pre-empted user input (which violates the trust of user input always being respected). I tried to reproduce this a minute ago on the slashdot.org front page, and it couldn't find the word I was typing in, despite me looking straight at it. That's a whole other flavor of broken, so I'm guessing that the "Find" functionality needs to go back in the oven.
To its credit, when you find a specific word, it will highlight its location in the scroll bar on the right. That's really cool.
Also, I'd love to see a themes / skin option, complete with a few selections of "anti-flattened" themes, for those of us who don't want to follow along with the design diatribes of Jony Ive or Julie Larson-Green. I like drop shadows and 3D raised elements; they help me locate controls in the window. I'd like to see a Vivaldi theme with 3D raised buttons, and with an option to disable all slide / fade animations (because those animations are a waste of time IMO).
That said, it's really good to see a proper successor to Opera, instead of that Chrome-clone joke that I unfortunately installed under the pretense of it being called "Opera". THAT browser is NOT Opera; Vivaldi is the true Opera.