@jon
Greetings!
I am a fan, what can I say, I'd been using Opera since it had ads, 8.5 afair, till the last presto version 12.18 became unusable and the standalone mail client way longer than that.
Obviously I jumped on Vivaldi right when I found about it. I am very happy that you recreated old Opera functionality (and then some), because that's what I wanted to get.
I love the mail client for its different paradigm - don't do folder filtering, instead use contacts + real time full text search. I had a very heavy business mail account 2005 - 2010, it was so fast in Opera, by the time my oultook colleagues found anything in their mail I was long done & gone making coffee. It was sad to watch sometimes.
One thing I miss is aesthetics. Opera had a very elegant UI (at least till Vista showed up with its fake glass). I udnerstand Vivaldi goes for 100% pragmatic look determined by technology and it's fine i guess. We could do way worse but I yearn for times of chunky icons and bars full of 3D buttons when we were young.
Also, features for tab hoarders on withdrawal (highlight duplicates in windows panel, highlight the same domain, stack duplicates etc).
Anyway I work in IT and install Vivaldi on most machines that come through my hands.