I wouldn’t go so far as to say Vistalites are “very visible” in the daytime. I have one that I hang on my Camelbak and I don’t even bother to turn it on until dusk. If you want it to have any chance of being visible in the daylight, you’ll have to change batteries more frequently to make sure they are fresh.
It was my primary taillight until I got the NiteRider taillight, so I’m not knocking it.