A robot vac works on random sweeps of a floor...with the hope that over the operating cycle it will cover all the floor. It however won't clean in corners as the robot vacs are usually round in shape leaving a corner (room, furniture, mats etc). They also can't walk over the edge of thick rugs and will take such rugs as an obstacle...thus not cleaning the rugs.
The other potential issue with robot vac is they are either the sweeping kind (still call vacs) or are the vacuum kind. The ones that do vacuum currently have less suction power than regular vacuums and are possibly best placed to remove light dust.
In my view, robot vacs currently don't replace regular vacuums, but may be used to assist in the cleaning of some areas in between normal regular vacuuming.
Would I buy one, no. This is because we have many obstacles and also rugs which would make the robot vac not very effective/assist in cleaning very much.