I use an APC 650 Back-UPS with no problems, PC left on 7/24, was working the other day and APC cut in for a split second about half a dozen times due to mains supply fluctuating, was reassured to see that all was working as it should. But if we're expecting a storm or there are long periods of inactivity, then yes I power down, just in case.
Have PC tower, flat panel monitor, scanner, and printer all into APC.
APC has a very good tech support dept, and would recommend them if you need a ups.
WinXP (stand alone) 2002 sp1.