One of the challenges we've had to deal with as our product surface area has grown, is identifying and reproducing bugs. We use Sentry for exception monitoring, however, it's usually difficult to try to reproduce bugs. I first heard about FullStory from our friends over at Flexport (check out the Stack Story and you'll hear them mention it: https://stackshare.io/posts/how-flexport-builds-software-to-move-over-1-billion-dollars-in-merchandise). FullStory let's you record user sessions, and play them back to help you identify bugs and UX issues. You're even able to view the console errors live as they happen during the sessions!
We were pretty blown away at how comprehensive the product was at first, and it seems to be getting better every time I use it. Only complaint is that it's super expensive once you're in the hundreds of thousands of sessions so we had to stop trying to record logged out sessions, we only use it for auth'd sessions. We also started out using it via Segment but once we needed to watch out for the number of sessions we were recording we realized that it was impossible to restrict FullStory recordings on a per-page basis without ripping it out of Segment, so we ended up just using their JS snippet and putting that in the Rails views that we wanted to monitor closely.
The ability to share specific portions of sessions, speed them up, skip inactivity, and all sorts of other little features all add up to a really solid product that helps both our PMs and engineers improve our own product much quicker. I officially requested a Sentry + FullStory integration a while back https://twitter.com/yonasbe/status/871987738777616384, still waiting on this! #UserFeedbackAsAService #reproducing-bugs #sessionrecording #bug-squashing