I got one as a christmas present one year, and have never used it for vacuuming. I have used it to deliver drinks at parties, as a cat toy, to run ethernet cables, and to make an animatronic facehugger for halloween, but never for vacuuming. They're silly as appliances, but great nerd toys.
You be nice. They're spectacular as appliances. You make sure all your furniture has legs longer than a Roomba is tall and you keep all your cords off the floor. Then you run them two or three times a week and the floor is spotless. Granted, we have no pets without gills. Everyone I've ever known to gripe about the ineffective nature of Roombas has cats and/or dogs. Having seen what it does with dust and humanhair I can imagine running a Roomba once a day would be about enough to keep up with a cat.
I had a roommate with a Great Pyrenees mix and he got a Roomba so he wouldn't have to vacuum so much. The Roomba lost hard. Not that the regular vacuum was winning though, that thing still had to be disassembled and unclogged every few months.