COSMIC is not so much “testing” as in “many bugs”, I basically found none. But they just lack maaany essential features. It is really breat to get a desktop that implements nice fancy stuff from Plasma etc. straight from the beginning.
I am not sure how ready it will be when it launches, as in features. But it is pretty nice and the apps are damn fast. I use the appstore on Fedora Kinoite when searching Flatpak apps. It has no native package integration so it does what I want, really nice.