I actually prefer manned check-out. It tends to be much faster. The checkout people tend to be wicked fast at swiping groceries in front of those scanners, much faster than I can dig into the cart, find something, fumble around scanning it, then place it in that little post-scan cubbie---ughhhhh. And manned checkout is already parallelized: Next customer queues up groceries while current customer is being serviced.