As soon as they get the mail client happening, I'm there. In the meantime I'm using Sea Monkey, which is OK. Hopefully I'll be able to import my old Opera bookmarks and address book into Vivaldi.
When Opera was first released they suggested using a different E-mailer as Opera's was so radically different than what was out there. How? Not sure, I've never used it, taking their word for it.
Vivaldi will import Bookmarks, Passwords, History and Search Engines (Selectable) from IE, Opera, Mozilla Firefox, Bookmarks HTML or an Opera Bookmarks file. I've always saved a .HTML and .ADR file when exporting (backing up)) bookmarks.
There is no requester asking for ones bookmarks, pressing Import hangs Vivaldi, when restarted it's found some bookmarks to integrate and I'm not sure from where. Of my Opera 12's 3330+ bookmarks only 227 bookmarks were shown to of been added. I haven't used Opera 15+ enough to save many bookmarks (speed dial). 227 could be links that still work, but it hadn't had enough time to check, nor shown up on HTTPNetworkSniffer.
You can't tell the difference between a link or a folder within a folder, so a real mess in that respect. Each bookmark folder is selectable as a bookmark bar, testing it for this post, the folders are shown on every view, hovering a mouse above a folder shows all the selectable links it contains (very kool).
I set it to be my default browser, seeing if "the Origin Game Plug-in" would work with it (the only reason I use Firefox). It won't -Vivaldi won't load, killing the Vivaldi process and closing Origin is required, so Firefox stays.
I've made a list of improvements and fixes (I feel required (ie: close all tabs)). This post fits well into it. It's a start but a ways to go yet.