I have been using it quite a bit over the last couple of months. There are still lots of features that need to be added but it already has the one I find most useful that Chrome, Internet Explorer and Firefox (other than an extension for version 3) lack and so I can live without the rest of the missing features until they have time to add them. The way I use web browsers the rewind button is essential and before Vivaldi the only browsers that supported it were Firefox 3 and Opera (version 12 and earlier)
The two most useful features I am waiting for are for them to make extensions properly accessible so that they can be configured rather than only being able to use default settings AND to have all bookmarks open a new tab rather than overwriting the current page.