I tried it, think it was pretty cool but of course buggy since it's alpha. Can't wait for where it will be in another year. I can see it replacing gnome in the future, but not plasma. Plasma has way too many features and is so far ahead.
I installed it on my gaming laptop. The OS is solid, probably one of the best distros for laptops out-of-the-box and COSMIC is pretty good but have some rough edges like problems with tray icons and games, so I also did a quick "apt install gnome-session". I'll switch over to Plasma 6 in a little while when it's ready, never was a GNOME fan but it's ok for a couple of months.
When playing with COSMIC I use Nautilus as COSMIC Files is still missing features like adding smb shares and file compress/extract. I'll probably be able to use the DE full time later this year.