I haven't to be honest, the couple of companies I have been with that used it have used outdated self hosted versions. Maybe the licensing scheme should encourage organizations to upgrade rather that stay on an old version.
I always tell people that Jira used to be terrible and has been drastically improved (in terms of usability, stability, and features) in the last 5 years.
It's still not perfect at all and is too flexible to be a safe choice for all orgs.
I've noticed the UI to be a bit schizophrenic. The list view has one way of displaying a ticket, but a ticket's details has another way of displaying the ticket. It's slowly becoming standardised, but I always have to hunt around to find the fields I'm looking for (often having to go into the details view).
You can customize that. It's genuinely crazy how deeply you can customize Jira (to the point where it's probably a bad thing)