My BX750MI-GR has saved my precious data on two occasions. Earlier this year, my seven year old, well used waffle iron shorted, popping loudly and sparking quite brightly in the process, probably because of all the gunk that had accumulated inside its less cleanable interior over the years. That not only made my breaker trip, but also blew the 16 amp main fuse. The UPS kicked in immediately, making a klick and a beep, which felt amazing. That was my first time ever.
As for the second time, see this post.
My god, perhaps once a week. I live in an area with shitty grid service, so because of my UPS I have uptime on my servers measured in hundreds of days