I don't use collocated repositories, so I can't speak to your second question, but I rarely hear people complain about it on their Discord.
Yes. Even with no UIs or editor support, it's already a better experience than git for me.
> Have you run into extra complexity and messes because of having two VCSes interacting in one working copy ?
For tools that expect git, you can assist them by using a colocated repository, so they can still see git. It works well enough, though most of the time, they will think you're on a detached HEAD.
That being said, I avoid git's mutating operations. I've heard that can cause trouble.
> Have you run into extra complexity and messes because of having two VCSes interacting in one working copy ?
Well, making git commits in a colocated jujutsu repository was a very confusing experience, because at some point I realized I wasn't on a git branch like I thought I was (after a few commits!). jj somehow magically did the right thing though and after switching back to it I had the commits I tried to make in jj and they worked just fine.