It really doesn't happen that often (despite living somewhere very rural) unless there are thundrestorms in the area (when I shut down & unplug all the computer equipment) - and as I'm now semi-retired I'm not working on anything *that* critical.
I've not used domestic (ie vaguely affordable) UPS systems since the mid-nineties when they were dreadful things with a nasty habit of destroying the power supplies of any PC unfortunate enough to be attached to one - I guess that put me off the whole idea
BTW - has anyone else noticed how much better PowerMacs are at surviving brown-outs than PCs? I guess apple use bigger capicitors in their power supplies.