I have an older (cheapo) UPS that I just replaced the battery on, which is probably over 15 years old now. It was much easier than I thought.

Coincidentally, right after I replaced the battery and left it to charge overnight, it saved my workstation from a power outage [events which aren't too common in my location] the very next day. It was quite satisfying to be able to save my work and shut down properly.

The old battery had stopped working long before replacement. So I guess the UPS was no better than a surge protector at that point.