Looks nice. I like that it defaults to vertical tabs, but I don't like the inability to give them any sort of hierarchy, be it a full-blown tree like Tree Style Tabs or with tab groups like Edge.
Since it's just a Firefox reskin, I was able to login with my Mozilla account and sync my extensions, Tree Style Tabs included. Enabling Zen's "compact mode" turns it into a pretty ideal setup for TST, since the default tab list disappears entirely. Still would be nice to not need TST in the first place.
Workspaces seem like a nice idea in theory, though currently underbaked in practice. Deleting a workspace appears to be bugged at the moment; attempting to do so just switches to the workspace instead of deleting it. It'd be great if they could integrate with Containers; I'd love to be able to e.g. set a default container for a given workspace (and then all tabs in that workspace would default to that particular container).
The Zen Sidebar is pretty cool; I can see myself using the premade Wikipedia and Google Translate panels quite a bit, in addition to setting up some of my own. Resizing the Sidebar is pretty finicky, unfortunately; dragging the edge will often fail and cause the Sidebar's size to snap to a tiny width.
Zen seems to have a built-in profile switcher, which is interesting, but seems redundant with Containers and way more limited. I also don't know why it defaulted to creating both "Default (alpha)" and "Default Profile" profiles, nor do I know why it set "Default (alpha)" as the default. I assume because this is an alpha release?
I don't know if Zen changes enough for me to prefer using it over normal Firefox (especially with Firefox's own vertical tab and sidebar improvements on the horizon), but I appreciate that this exists and look forward to seeing where it goes.