I recently switched to COSMIC from KDE Plasma as I wanted to potentially upgrade my security at the cost of usability. I've been using KDE Plasma for a while, as I found it to be a powerful and pretty sleek desktop environment GNOME is also very nice and comfortable for me, but I was using hardened_malloc at the time so using it would break my system most likely. Window managers are nice too, but they require more time and effort than I am willing to put in, so I stuck to using KDE or GNOME.
I started using COSMIC on my latest Fedora install after my setup broke, as it seemed pretty interesting, and it was written in Rust. I am aware that COSMIC is currently lacking thumbnail sandboxing, although a solution is in progress (https://github.com/pop-os/cosmic-files/issues/1216) but as far as the entire DE goes, I believe there shouldn't as many potential memory leaks as C/C++-written software.
I am happy with the amount of power I have over my DE as a whole including theming, and I am even more happy over how easy it is to track with things like chezmoi. The keybindings are also really nice, and whatever I miss can be added back and tracked easily. The apps are pretty usable, although I've had to make an issue with the media player and am soon about to make another one. Overall, a pretty good experience all things considered.
Hopefully, my issues will be sorted out in future releases, and I am grateful that a stable release even exists now. My blessings goes out to all the developers who have worked on COSMIC. Here's to another great release!