Jan de Mooij and the Mozilla JS team just used rr to figure out a GC-related top-crash in Firefox Nightly. The IRC discussion starts here.

I think rr is great for most kinds of bugs, not just the really hard ones. But obviously hard bugs are where there's the most win to be had.

http://robert.ocallahan.org/2016/02/these-bugs-are-impossible-to-fix.html