The killer app for cloud IDEs is honestly going to be something GitHub code navigation. Imagine being able to do PRs with click to definition, find all references, etc., without having to pull in the changes locally. It's also great even right now for reading open source projects when you are trying to understand how something works.
They are great for viewing code, but I'm not convinced they are good for writing code.
Reviewing PRs on GitHub is embarrassingly bad, given that it's their core feature. It hasn't been upgraded for years, even, missing features BitBucket/Stash had like 10 years ago. Now a file tree is finally in beta, so it's possible to navigate between the files. But it's still impossible to see the context around changes without having to press expand/(...) a hundred times.
I've recently experimented with the PR feature inside JetBrains IDE's. It allows one to navigate quite well without having to actually switch branch. Like it so far, nice to see the whole file and related files, not just the diff.
TBH reviewing PRs on VSCode's Pull Request extension works pretty well for me.
I agree it is a bit slow but I get to see the changes with all my keybindings, themes and other tweaks, this just makes the experience better IMO.
They are great for viewing code, but I'm not convinced they are good for writing code.