Errrr... Your having flashbacks to the old days...

Most modern UPS's either use a stepped wave (cheap ones)
or the more expensive units do make a sine wave. These
sine waves can be cleaner than what you get out of the
power grid.

Think about it... The square wave would be a problem with
transformers... In modern computers they ALL use transformers
to convert the power down to 5v and 12v. They dont make UPS's to
blow up computers, but to save them from power spikes and
voltage dropouts. One side you can look at though is that
square sine waves do their damage over the long periods of time.
Almost any UPS can work well for a little while.

This makes me want to look at my APC ups now.... Geeez