As far as I know Sainsbury was the first to go down this route where you scan in your shopping and do the payment yourself.
Now it seems B&Q have also decided to install self-service (no service more like) checkouts. They have one normal checkout with an operator and then a few members of staff top "help" when the self-service ones go wrong which they do all the time.
No one I have spoken to likes them and I no longer shop in Sainsburys as a result. If the queue is too long at the one manned till then just leave your trolley full of shopping and walk out. They will soon get the message!