I would say quite usable if you are confortable doing rpm-ostree update via the command line while we work on rpm-ostree support in Discover. I am using it regularly on my main laptop.
There are currently no KDE Apps available as Flatpaks from Fedora. I’m working (with others) on packaging KDE Apps for Flathub first as they get “day of release” updates to latest release. I will work on packaging them as Fedora Flatpaks next. In the meantime, for missing apps, you can overlay them from the official RPMs from Fedora repositories.
If you want the comfort of official Silverblue releases, you can also overlay the KDE Plasma packages on top of Silverblue (see the (outdated) list of packages) and switch from GDM to SDDM as login manager.