UPS units do actually stabilize voltage and can correct over & under voltage conditions to a certain extend... if voltage goes outside of the range it can compensate for, then the UPS will shut down.

In my area, there are lots of short power disruptions (just a few seconds) and in the summer there can also be brownout conditions with low voltage.  Having a UPS unit goes a long way to keeping systems stable when the power blips out for just a few seconds (or even just a few cycles).  Most of the properties I work at also have power generators so the UPS units in each rack keep everything running for the 20-30 seconds its takes the generator to start up and there's no disruption to systems.

The Wattbox/UPS combo seems interesting with the ability to turn off specific loads after a certain duration to preserve battery life.  OvrC alerts for power out and battery condition will also be useful.