I'd add that the UPSs I have purchased in the past have failed more often than the computers I had plugged into them. No more UPSs for me, thanks.
I have two APC UPS devices. One for my desktop and one for my other accessories (cable router, switch, Vonage router, and phones). My area has short power outages every so often when they are working on lines, etc. Plus, using Vonage, I need to power my internet devices for phone service during power outages. I also have my cell phone and could use it as an emergency battery for charging as well.
I've had both for about 4 years now with no problems or issues. I have had to replace UPSs (or the batteries, depending on which costs less) as, like a car battery, the batteries last between 3 to 5 years. Most UPS that fail tend to be either under powered for the devices that are plugged into them or people don't maintain the batteries. Running the batteries down too low will damage them. That's why you are better off configuring your devices to shut down after being on battery for about half the run time. Mine is 13000VA and I get about 20 minutes of run time with my PC (300W during normal usage). I also run the APC battery software which checks the batteries once a week.
APC does have some crappy low end units but the Back-UPS XS models have been reliable for me.