Their infrared sensors help to get them out of tight spaces, find dirt, and even keep them from falling down the stairs. They stop cleaning when their bin is full and return to their docking stations when they are finished.
Consumer Reports' testers pitted the three robots against measured patches of cereal, sand, rice, cat fur, and paper bits. The iRobot Roomba 750 did better than the others simply because it covered the all areas of the room several times. Other models are more systematic, moving in straight lines across the room. They pass over an area once, so whatever litter they miss stays there.
Consumer Reports recommends the iRobot Roomba 750, about $450. It did an excellent job in all of Consumer Reports' tests. It took a little longer to do the room than others, but if you're not there doing the work, what does it matter?
However, Consumer Reports points out that a robotic vacuum can't replace your regular vacuum for deep-cleaning carpets. If you are in the market for a regular vacuum, Consumer Reports named a bagged upright from Sears a Best Buy at $200. It's the Kenmore Progressive 31069.