We have an iRobot Roomba in the shop as a gimmick, and another one at home. I am sick of fixing them actually, only this week the one in the shop needed a new battery which I think had fried because a diode in the charger had fried. The one at home has decided that its' side spinning brush doesn't want to spin any more.
On carpets they are not terribly serious. On laminate floors they are great! The one at home is really cool on the hard floors for dust and fluff collecting; it goes under the TV stand and some of the furniture too. Really, they are just a talking point that is also quite fun.
Yes, Dyson did briefly do a robotic one at about £2500, called the DC06 as VCH said. It looked like this:
It was soon canned and here is the interview with James Dyson talking about it: James Dyson discusses the Dyson DC06