Been using Vivaldi almost exclusively for more than three weeks now …
... and I have to say I really like it. I'm coming from Firefox, which I have used for more years than I care to admit (even had a different name back then). But lately Firefox began to hang up and stutter a lot after it had been opened for too long, even with not too many tabs opened. I didn't like Chrome, which I tried on several occasions, but I can't put my finger on exactly why not. I stumbled across Vivaldi quite by accident, decided to give a whirl, and here I am. It really didn't take much to get used to, and importing my bookmarks from Firefox was a snap. Everything imported exactly how they were set up in Firefox. I really like the menu system, the sidebar and tab stacking but I haven't gotten into too many other things yet. Most of my addons, which I got from the Chrome store, but at least one exhibits quirky behavior - Lastpass. I attribute it to being optimized more for Chrome. I'm not a big fan of having two tabs open on startup (start page and home page) but I got around that by having it open where it closed. It seems to disappear to minimize whenever I update my graphics drivers (nVidia GTX 960) and sometimes after Windows 10 updates, and they won't maximize right away. It usually comes back after I fiddle with it for a while, but I'm not sure why. Not a big deal. I figure it's a known bug and will fixed sooner or later. Vivaldi is fast, hasn't locked up on me and seems to render all the sites I visit just fine. This browser is seriously good. I cannot wait to explore it more as I go along. I'm about 95% ready to use it as my default browser, and I'm sure I will after a while. It's pretty much the only one I use now, except when I need Lastpass for a number of things. I am very pleased so far with Vivaldi.
And to make matters better, Lastpass is working in the most recent developer builds of Vivaldi!