All self-checkout systems I have used have horrible user interfaces. That's why they get taken out -- the customers hate them.
The voice and the 'dialog' on the ones at Safeway and Lowes drives me insane. It is both irritating in tone, and infuriatingly condescending. I don't need a machine to talk loudly at me like I was a five year old in a snotty teacher voice like it was talking to a barely sentient animal. Plus, the work flow assumes that you scan, pile up, then bag your purchases in store provided bags. If you are in a municipality that requires that you bring your own bags, it breaks the workflow and the ****ing thing nags at you loudly until you do everything its way, regardless of how inconvenient or inefficient it is. I don't like being essentially yelled at like a bad puppy by a machine.