fireberd
A UPS is a must for me, a "power conditioner" won't get you the backup needed. Power outages, especially the momentary type are killers on PC's. Anything in the PC or peripheral equipment can "fry". A good UPS will help to eliminate 99% (nothing is 100%) of the problems.
Well, there are "power conditioners" and there are "power conditioners".
An $8 power conditioner from Walmart is not the same as the higher-end power conditioners (ie. Furman units that cost $500+) that will protect your gear through most brownouts and spikes.
And no, not even the better PC's will keep your system up if the power goes out - that's not what they do - but they will protect in a much more efficient way than any of the pro-sumer level UPS's.
Ideally, you will have a quality power conditioner with UPS.
As I stated earlier, we don't get many outages here, so I haven't found a need for a UPS, but I'd trust my Pro3500 to take a bullet (surge) for me over a $50 Office Depot UPS any day of the week.