gnolam: TBF, as a customer, I'd be Real Annoyed if I used self-checkout and found I'd been double-charged for some item(s) because, while juggling items in preparation for checking out, things got scanned and I didn't notice. I'd much rather have a definitive 'I am here, start scanning' button.
Every time I've had a bad experience with the self checkout machines, I've gotten management involved and made them suffer through the same problems; they seem to get tweaked within about a month or so after that, depending on how awful they are :)
I don't prefer them, but when they're set up and configured correctly they can be a lot faster than waiting in line while someone shares their life story with the cashier