I installed an APC 1500 Back-UPS uninterruptable power supply on my desktop computer yesterday. We get not-infrequent brown-outs and even the occasional power outage in F'burg.
Setup was a breeze. The only trick was needing to connect the battery, which came pre-charged (thereby causing a discharge arc when connecting - I was not warned in the manual and it took me by surprise). The included PowerChute Personal Edition software installed easily on my PowerMac Quad G5. I was pleased to see that I now have a battery icon in my Finder bar and it works to indicate the status of the battery.
The G5, a 23 inch screen and a 17 inch screen, my laptop power supply, Klipsch speakers, a USB hub and an external hard drive add up to about 300 W, giving about 15 minutes of power from the battery. When the machine is sleeping and the monitors are off, the power usage drops way down.
I tried the obvious test by unplugging the power cord. The UPS functioned as expected. The alarm (4 beeps per minute) sounded and the machinery stayed up. Oddly, I did get a dialog box warning me that a USB device was removed and not properly dismounted first. I think that came from the USB monitoring cable on the UPS, but am not certain.
The only (very minor) complaint is a faint smell of ozone. I think I would rather get used to the ozone than risk losing a HDD, so I'm happy.