AskAlice
I always use the Scan As You Go facility at Tesco and Sainsburys - so much easier with a large shop. Even if I have to wait for a member of staff to verify alcohol or other "over 18" items or have a security check, it is so much quicker than queuing at the manned tills, unloading the trolley and then re-packing it as it all cascades down the conveyor belt and falls over! I realise that the manned tills are better for some shoppers, but not me...
Looking at the trolley load self scan in Sainsburies, it has a large flat area with no sides on which you put your trolley contents, with all the problems of things sliding or rolling over the edge, and the space looks quite small so probably won't take all a trolley's contents. That means you need to get a second trolley the other end to load into - except that first, before putting them in the trolley to take to the car you have to put them on a small area, smaller than the unload area once scanned and if you try and take anything off that until you have got everything on it, the bells and whistles sound. using it would be a nightmare.
As it is on those weeks I have a trolley laod of shopping, I usually shop early so I walk up to a till with an operator, have a large enough space to put all my shopping on,. It has raised edges so things do not slide off. I then push the trolley to the other end of the payment area, set up my bags in the trolley, a matter of seconds, and as the till operator puts the things through the till, I pick them up and put them in the bags in my trolley. Quick, simple, and no double or, even, treble handling.