In the land of the free...
or more accurately overly free waistlines...
I have never had issue with them, most of our local stores have them and are really useful for quick buys, however if a cartfull of groceries it is better to see a real person, or, if you have contraband..erm...alcohol, which is pretty much every store trip for me (why they cannot simply scan my drives license... oh wait yeah I could have used someone else's silly me!)
Even the large bulk stores like Costco have them, and can be really quick, though it is fun when your purchase falls backward on the conveyor and breaks the magic eye thing again then the machine gets confused and the red light of shame flashes above your station.... there is usually a happy chappy (or chappette) to fix it fast though..
Home depot (B&Q) have the same things and never give me grief I think they disable the weight thing for a good reason, though there is often a "loss prevention officer" present to check your receipt if they think you are doing a number on them, I tend to glare at them while clutching the 3ft long receipt it spits out for a bag of screws...
as for the human operated machines, I always scan my card and enter appropriate pin well before the person has finished ringing up my inventory, then all I need do is smack the green button at the end and I get on my way (I hate the people who either wait till the end and then rummage for the card in a purse or bag, knowing full well they will be expected to pay at the end of the whole thing, or those who do this, then decide to get out their bloody chequebook! I mean c'mon! you have been standing there for 5 minutes watching this person scan your shopping and only at the end of it do you think "oh my! I need to find the method from which I am to pay this hardworking person, now where could I have put it?"
S.W.M.B.O when she used to pay with a check had it out and ready ( and before the days of the cashier being able to fill all the necessary "pay to:" crap in she had it waiting for a total... though she mainly paid in cash.(.A method of payment that can flummox most under the age of 30 when you give them pocket shrapnel to make the change a whole number like $5 )
maybe it is the way America does it, but they seem to be more together than the shopping hell that the uk seems to have now
btw what happened to the Sainsbury scan while you shop thing? I thought it was quite cool myself