It's critical to our eng, design & product processes.
We switched from another tool because the other tool was painfully slow. It was a night & day difference. It's been about a year now since we've fully committed to Linear. I'm very glad we have!
We use their cycles feature heavily. We ship our app weekly, and our product development process is split into three 2-week cycles that form a "milestone". We model that work accordingly in Linear.
We also use their "Projects" feature quite heavily. We'll often have folks working across a few projects a time. This feature helps my ADHD brain understand what the most important thing across any number of ongoing projects is, and then focus on accomplishing that.
Personally, the best feature for me is the speed of the app. That, alongside the keyboard shortcuts, make it pretty hard to give up.
Another thing I particularly like about Linear (aside from speed, product polish & wonderful design) is how you can opt-in to using it more. If you just want to use it as an issue tracker for teams, that works great! It's how we started. But you can also grow into the product more as you harden your processes or figure out what works best for you.
tl;dr — I can't imagine ever using another tool to help build products besides Linear.