Robert Heller suggested that UPS architecture matters: AC->DC::DC Batteries::DC->AC Where input AC is electrically decoupled from output AC. Not many adverts for UPS's explain whether this is the case with their UPS.
APC's SmartUPS line, Liebert, and Eaton Powerware are all true-sine wave UPS's, and do proper decoupling. Unfortunately, this kind of data doesn't make for great ad copy, so it's left out, and you have to dig deep into datasheets to get that information. I pretty much only use APC, and we have truly crap power here. Because of some heavy industry in the area, brownouts are common, and that'll kill a PC power supply better than anything. I've pulled one 7 year old APC from a server closet where the lightning took the top of the telephone pole OFF. THE UPS was fried, some of the breakers in the building were fused (!), but the servers were fine, outside of the router that got zapped from the DSL modem.
I beg to differ about APC. The accepted term for what Robert described is a "double conversion" or "online" UPS. APC's SmartUPS family is only available with the double conversion feature if you specify a "SmartUPS Online" model. The rest of the SmartUPS family use "Line Interactive" which runs on mains power until the voltage/current/frequency goes out of tolerance, at which point they cut over to battery. The Liebert GXT2/3 family which we use quite a bit of were, until recently, strictly double conversion.