I have the irobot pet series and i love love love love love LOVE it. It does go in random directions....but it has a method to it's madness. It has the ability to "sense" dirtier areas and spend more time there.
It does require some maintenance....cleaning it weekly and emptying out the catch once a day. But....as busy as I am, It does a better job at vacuuming than I ever could. Since I rarely find the time and my Rumba requires a simple button push as I walk out the door.