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.
They scream at you when they lose wall power. You know that they at least prevented a hard power off when that happens.
Yeah, but no guarantee that the hard power off would have caused data corruption, especially with a relatively idle server, which all of mine are most of the time :/