As I said, they are not a substitute for a "proper" vacuum cleaner although ours (an iRobot Roomba 866) does pick up quite a bit of dust and fluff. We usually set it off on is travels when we go out and come back to find that it has cleaned up and taken itself back "home" to recharge.
It cleans reasonably well in corners as, although round, it has a flexible, circular brush on the side.
It cost nowhere near £800.
It does not affect the intruder alarm.