This night-vision camera shows how they work. Their infrared sensors help to get them out of tight spaces, find dirt, and even keep from falling down the stairs.
They stop cleaning when their bin is full and return to their docking stations when they're finished.
But how well do they clean?
Consumer Reports' testers pitted the three robots against measured patches of cereal, sand, rice, cat fur, and paper bits.
"Check out this time-lapse photo. The Roomba did better simply because it covered all areas of the room several times," John McAloon of Consumer Reports said.
Other models are more systematic, moving in straight lines across the room. They pass over the area once, so whatever litter they miss stays there.
Consumer Reports recommends the iRobot Roomba, at about $450. It did an excellent job in all of Consumer Reports' tests.
"It took a little bit longer to do the room than others, but if you're not there doing the work, what does it matter?" John McAloon of Consumer Reports.
Robotic vacuums are easy to program to tidy up your home while you're away. But they can't replace your regular vacuum for deep-cleaning carpets.
If you are in the market for a regular vacuum, Consumer Reports named one a "best buy." It's the Kenmore Progressive with the model number 31069 from Sears.