I've mostly used them on new circuits, but have put them on a few of my older circuits too. For my small sample size, I've yet to have one nuisance trip.
Yeah I've replaced all of the breakers (to living areas) in my 90s home with AFCI ones. Haven't had any nuisance trips. If I had "nuisance trips" I'd be more worried about my wiring than about my breakers...
