I've been using it for a few days so here's my early feedback for you:
It's fast. I've had no significant speed problem so far even when having a lot of tabs and workspaces at once. The only speed "problem" I've noticed so far is that it sometimes takes a second or so to switch between 2 workspaces. The team seems to have a focus on optimization and they even have benchmarks comparing zen to other browsers on their website and github.
It's based on firefox. like librewolf or mullvad. So on the technical side it should be very similar to firefox. This also means you can install any firefox addon.
They seem to have some good security features and since it's firefox based you can install security addons on top so it's probably pretty good I think.
As someone who keeps a lot of tabs open at all times it's the browser with the best tabs management features I've ever used. There are workspaces so you don't have to open multiple windows to organize your tabs, there is a preview feature that allow you to open a link without opening a new tab, there is a split view feature that allows you to open multiple pages in the same window at once, etc...