Since you made me curious, I just skimmed through the features of Vivaldi:
- It’s your web. Surf it the way you want: Fine.
- Powerful, colorful History: Certainly a nice feature. I’m sure I’ll use it once.
- Write Notes: Why on earth should I want to write notes in my Web browser? That’s like advertising a car whose feature is that it comes with a hamster in the trunk.
- A million ways to customize everything: I’m already spending way too much time with customizing stuff.
- Tab Stacking and Tab Tiling: Hmm. In Safari I hit ⇧⌘D and the page is out of the way but still quickly accessible. And it doesn’t use memory. And it’s on my iPhone.
- Choose your style with Themes: See above, customizing.
- Quick Commands: Well, looks nice. But, if I want it I can get the same via KM’s List Prompt, or via LaunchBar.
You see, I’m a bit negative today 
BTW, I’ve seen that it also contains a “Task Manager”. I opened it and the first thing it did was trying to connect to Google. Alright…