I prefer them. There’s trade-offs (like disk usage and occasional theme issues) but it’s worth it to me for the sandboxing and ability to easily run a newer version of an application than your distro has packaged up in their repos. It’s better for developers since they don’t have to support deb, rpm, etc. etc. And long term, it’ll allow immutable systems to become the default and that’ll be good for security and stability.

Between Snap, Flatpak, and AppImage, I default to Flatpak. It seems like the best supported even if they all have their strengths and weaknesses. AppImage is great for old versions of software you don’t want updated/integrated into menus. Snaps are basically the same and I happily use them if there’s no Flatpak but it’s so tied to Ubuntu/Canonical that some people have opinions about using it. I don’t know of any developer stubbornly refusing to support Flatpak on ideological grounds.