Just don't...
Seriously though. I bought an irobot roomba a year and a half ago and it's been accumulating dust. I've used it a dozen times but you have to use it very often; otherwise it will get stuck with large chunks of dust every time you run it and you'll be chasing after it emptying the bag and restarting it... At the time I was hesitating between the roomba (vacuum cleaning) and the scooba (floor washing) but if you really want one now I would go with the floor washing version (if you have hardwood, marble, tile, etc..)