Recs.
Updated
Vivaldi is a Chromium based browser with its UI written with web technologies (HTML/JS). Vivaldi advertises itself as a "power-user browser" as it tries to bring features built-in reducing the number of extensions needed.
Vivaldi was founded by Jón S. von Tetzchner, creator of Opera browser and its CEO until before the switch to Chromium.
SpecsUpdate
Pros
Pro Tab behaviour can be customised
The order you toggle, open, close or clone tabs can be modified to match a workflow that best works for you. For example you can choose tabs to toggle in recent order so you can go back and forth between the last most useful tabs without needing to change their position on the tab bar.
Pro Updates via dialog box
Unlike most modern browsers (but like most browsers of yesteryear), Vivaldi gives you a dialog box when an new version comes out; it doesn't just update silently in the background. The dialog box tells you about the changes that have been made, and lets you chose when you want to update.
Pro Allows navigating to the next page without searching for the link
A feature called fast forward puts a dedicated button before the address bar that is designed to help you navigate to the next page of a multi-page article, forum thread or search results without having to hunt for the link.
Pro Cross browser extensions with Opera and Chrome, Firefox web-ext's too
All Vivaldi, Opera and Chrome users can get extensions from all extension/addon sites, especially from Opera Addons and Chrome Store.
Pro Add websites as web panels for easy access
You can add sites as panels for easy access them. It allows reading two websites side by side with a simple click on the panel button. Useful for programming language documentation manuals like from DevDocs or Mozilla Developers. Panels can be hidden when not in use.
Pro Custom search engines
You can add as many search engines as you like. For example you can set up one for YouTube, one for Wikipedia, or whatever you want: you need only the URL of the search and it can be used from the URL bar (by prefixing it's nickname, yt
for YouTube for example).
Cons
Con Proprietary
While Vivaldi is currently available gratis (without monetary charge), it is currently not fully libre (meaning that it does not allow users to view the source code used to create, to modify that code, or to redistribute modifications) and is therefore neither free nor open-source software.
The C++ source code however is open-source under a BSD license and can be downloaded and browsed from here.
The UI code though is not open-source, but it's easily readable as it's HTML, CSS & JS. Modifications can be shared as their forum even has a dedicated section.
Con Comes with many Ads
After installation the first thing you have to do is to remove all the default bookmarks and quickdial links which is really annoying. There is more minor stuff like the default search provider or your start page things that could be easily managed with a wizard at the first startup.
Con No silent background updates
Unlike most modern browsers, Vivaldi doesn't have the option to update silently in the background. It gives you a dialog box when an update comes out. Worse, the box doesn't warn you that not updating the browser could lead to security risks. That could be pretty bad for your average joe, who doesn't know computers that well.
Con Closed source
While Vivaldi is currently available gratis (without monetary charge), it is currently not fully libre (meaning that it does not allow users to view the source code used to create, to modify that code, or to redistribute modifications) and is, therefore, neither free nor open-source software.
The C++ source code however is open-source under a BSD license and can be downloaded and browsed from here.
The UI code, though not open-source, is easily readable as it's HTML, CSS & JS. Modifications can be shared as their forum even has a dedicated section.
Con No option to open saved tab stack as... tab stack!
You may save stack, but you can open it as separate tabs. Interesting that when you save session with stacks, you'll be able to open the stacks with inner settings of page tiling (size and zoom) as well. Contradiction worth of shame.