Those self-checkouts were popular here in the States several years back. Most of the Wal-Marts started using them. It was a large failure, so that has since changed. Most of the time, someone would be assigned to stand at a front-side podium in case assistance was needed (assistance was always needed), but that same person was also in charge of deterring theft. This created problems, obviously. The self-checkouts have been mostly phased-out due to theft, I believe. I believe, at one point, they even had someone check your receipt against the items in your bag to make sure you scanned everything. What a total mess. The convenience was obviously lost at that point.
This was one of my pet hates in the UK. I just can't stand those damn machines, and the computer that tells you time after time that there is an 'unknown item in the bagging area, call for assistance'. It's really, really infuriating to be forced to use them and I really can't see the point if there has to be someone on hand all the time to assist. It's absurd. On the annoyance scale, it's right up there with being caught in an endless cascade of sub menus and more sub menus on automated call handling systems.
I noticed the self-service check-out machines at Waitrose were avoided by all (as they are at Central in Bangkok) but WH Smith at Heathrow force you to use them. What a freaking hassle! Your hand wobbles and your item gets scanned twice and you have to cancel everything and start over. Unsurprisingly, they have someone stationed near the machines to help everyone. This is progress??