I have never tried the JetBrains' alternatives (Youtrack, Space, etc) but considering everything from JetBrains is usually very good, I'd like to hear from someone who has.
Used Jira at the last company. Use YouTrack now. YouTrack is better, hands down. Much faster UI, more keyboardable, better integrated into the IDE, integrating it with CI and mailboxes is easy.
Caveats: new job is small so it's a small instance. JetBrain's own YT instance is pretty fast though, so I guess it can scale well. Also, we don't use the new KB/wiki feature so can't comment on that. It's probably got less features than Confluence. We don't use the scrum features.
Biggest caveat - I speak as a developer. One of the biggest problems with Jira was the culture that surrounds it. Clueless PMs who had never written software in their life kept creating fixed workflows in which you couldn't transition tickets to arbitrary states (for no apparent reason, there seemed to be no benefit to these rules). So we were constantly being slowed down by the need to get the handful of people with Jira access to edit the workflows and add more transitions, which was pure makework, especially as even figuring out who the admins were was remarkably complicated. PMs also had a weird Jira fetish, they were like, if it's not in Jira it doesn't exist. So they forced the project to change from a more normal SW dev project early on to mandating that every commit had a ticket associated, and that ticket had to be scheduled into a sprint etc. It was just rank stupidity. Want to do a quick refactoring, as part of your current task? Roll it all into one giant commit because otherwise you'll need to file a ticket. New place doesn't work like that. YouTrack is used more simply, like a todo list. Git logs are the source of truth.
I haven't used them for real but (briefly) evaluated with the intention to maybe replace Jira, about a year ago.
Space wasn't there yet. IIRC the main issue was lack of a way to set custom fields via API, or some weird restriction around that. My general impression was that it was promising but too immature at that time.
For any Jira competitors reading this - start with providing 2-way sync between your product and Jira instance. It would make moving away from Jira soooo much easier!
The current state is that even importing Jira data is below what I'd expect - e.g. Linear (last time I checked) didn't import comments at all and all issue IDs were changed to new ones. Really, if there's FOO-123 in Jira it's table stakes for it to be also called FOO-123 in the new tool.
I wrote a tool to migrate an issue tracker to Jira which imported everything as expected and updated links in issue descriptions/comments to keep their integrity. It took few days for a one-time migration so I'd expect someone who builds a business in this area to just have that as a baseline. It's sad how bad the "import from Jira" journeys are.
Jira has a search bar which seems ok-ish. Except that I have to wait 10s after I open Jira for the search bar to start working because it's probably the last thing to become interactive after a page load :|
