There are "always on" uninterruptable power supplies that can provide the best power to you computer, but they'll cost more.
Most APC units will only switch over to the internal inverter running off of the battery when the wall power fails or surges beyond the preset limits. This WILL cause a short interruption or spike in the power, but in a good unit it will be fast enough that it most likely won't even be noticed by your computer. The downsides of this include the fact that the wall power quality may fluctuate and not be quite bad enough to trigger the UPS over to battery power, and that can cause system instability or actually hurt your computer in the long run. A really big power surge may also get through this kind of UPS before it switches over to battery power.
On the other hand, the always-on ones only use the wall power to charge the battery. The PC is always supplied by the inverter inside the UPS, which always runs off of the battery. This means that you always get pure conditioned power, and there is zero spike or sag when the wall power fails. A personal gripe I have with most of these units is that I can hear the inverter buzzing away, and that bugs me. They also have a slightly higher power draw than you'd otherwise get when just plugging the computer into the wall, since you're converting to DC to charge the battery, and then back to AC to power the computer.
In both types, periodically testing the battery (use a lamp, TV, or vacuum cleaner to test the battery, NOT your computer!) is very important because after the battery degrades, you're really just using a bulky and expensive surge protector.
