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.
(I reproduced the text prediction bug just now; the cursor seems to jump back on a prediction hit, but the lag between the start and end of the jumpback seems to be dependent on the complexity of the rendered page; perhaps it's Javascript complexity. This is not good, because heavier pages will always exist, and will always behave slower.)
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.
One other thing that Vivaldi so desperately needs is tighter control on content. These days, browsing on websites is a nightmare of cross-domain Javascript scripts, to the point where users are no longer rendering a document, so much as involuntarily debugging a Javascript programming mess. (NoScript listed 76 blocked scripts when I went to an article from The Verge; Vox loves them some web fonts and crazy canvas effects.) Back in the day, Opera gave the user a lot of control on how to parse CSS, whether to play embedded sound (the old "BGSOUND" tag), whether to load images, and so on. Vivaldi has the "Block images" switch (but unfortunately forces a page reload when its state changes; IMO it should not force a page reload, but should instead leave loaded images cached, or hide all images). Vivaldi could use a modern CSS hackery option akin to Stylish, but I'd really love to see something like NoScript used instead. And of course there's add-on selective block, but hopefully that will become less of a problem once I uninstall Flash.
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.