Fibre Flares get through batteries quite quickly and I don’t think many riders think to test rear lights – I’ve seen so many cyclists out with barely visible lights cos they simply haven’t bothered to check.
My FF mounts onto my bag where it’s nice and high up so a bit more eye catching to bus/lorry drivers. They don’t like getting a soaking though, a mate had one on his seatstay and it died after only about 6 weeks from being used in the rain. The rubber end caps aren’t impervious.
Oh, for batteries, I buy AAA and AA in bulk from 7dayshop, they’re really cheap (although they don’t last as long as Duracell or equivalent).