I'm still a current user of Opera Presto - that's what I always view CFC in these days - so I downloaded the technical preview as soon as it came out. It should be noted that it is not built on Opera, but on Chromium. The founder of Vivaldi is one of the co-founders of Opera, who was booted out in 2012 (shortly before Opera switched to Blink).
I like what I see so far. The shortcuts and their configurability, including single-key ones, is a key feature for me (pun not intended), that no browser other than Opera Presto can match. I still like Opera Presto's speed dial with the Ctrl+# shortcuts (who uses a mouse to switch tabs in a web browser these days? only half-joking), but I voted for that in the Vivaldi feature poll. There are some UI speed issues, though reading the latest dev build notes, it looks like they're working on that, and hopefully by Technical Preview 3 it will be better. And despite the rough edges, I think they're coming out of the gates as a power user's browser much stronger than Opera Blink.
I do hope it's able to gain momentum, and I think it has a good chance with the type of user who used Opera Presto, or would have used Opera Presto were it not for the occasional glitches due to lack of developer support for Presto. The main question may be whether enough of those users can discover it quickly enough to sustain Vivaldi. They've got a decent-sized team, and while I'm sure they expect to be bleeding cash this year, eventually they'll need to be self-sustaining.
For now, it's approximately tied with Firefox for my second-most-used browser at home, with Opera Presto still taking the lion's share. I haven't used it much at work since it doesn't have dev tools; I still use Opera Presto a fair amount there, too (and make sure our product works on Presto. Dragonfly's a nice plus at times in its own right). Even without that, I could see it taking over the non-debugging tasks that I use Firefox and Presto for currently.
