I have one of
these that I picked up for $25 from an office going-out-of-business sale. (Eventually, it needed a new battery, which was about $100.) Combined with one of
these for my two monitors, I can power my Mac Pro, RAID tower and all peripherals for 45 minutes during a power failure before I reach the last bars on the batteries. I think they would go 50 minutes before running out. I experience enough power fluctuations and failures that it's well worth it. I can think of at least a dozen times I've kept on working while the house lights flickered or went dark.
Of course, regular saving and backup is key for critical work, but I think a UPS is required for a desktop system, unless you work at NORAD or someplace where constant, clean power is assured.