Here in San Diego we've had self-checkout systems in most major stores for a few years now.
At first they were awesome because nobody used them, so you could zip right through and avoid the lines. But now, after they've been here for a few years, a lot more people feel comfortable using them but many of those people use them poorly and far too many people will use them even when they have an entire cart full of groceries. That's an entire cart full of stuff they have to scan without accidentally voiding the whole order (which I've seen happen multiple times), a lot of trial and error on trying to figure out how to scan their produce, etc, etc.
It has reached the point where I rarely bother with self checkout anymore unless there is absolutely no line for it because while the system works great when nobody else is using it, it doesn't scale that well because most people are terribly slow checkers relative to dedicated workers doing that job.
I've seen similar things in my local store. They've finally upgraded to newer self-checkout machines, which means there's less hassle. The worst with the machines is them always telling you "please put the item in the bag" because it can't register a gravy packet and it keeps telling you until a worker overrides it. Then there's always the fuck ups where the item doesn't scan, you put it in a bag and have to pull it back out, and then it tells you "please put the item back in the bag" and won't let you scan it so you can.
The one problem with the theory described above is that most walmarts have a 6 cashier express with a single line, and besides 'rush hour' when the store is packed it's often faster to go to a till to get <10 items scanned. I've ran in before and been out in minutes by going through a regular cashier and avoiding the express because the single line holds the problem that people are stupid, don't pay attention or can't hear and hold up the line, then they have to unpack their cart.
Theory is great, but it's useless when you're stuck behind a bunch of old women who can't hear them being called and spend 20 minutes chatting with the cashier.
At first they were awesome because nobody used them, so you could zip right through and avoid the lines. But now, after they've been here for a few years, a lot more people feel comfortable using them but many of those people use them poorly and far too many people will use them even when they have an entire cart full of groceries. That's an entire cart full of stuff they have to scan without accidentally voiding the whole order (which I've seen happen multiple times), a lot of trial and error on trying to figure out how to scan their produce, etc, etc.
It has reached the point where I rarely bother with self checkout anymore unless there is absolutely no line for it because while the system works great when nobody else is using it, it doesn't scale that well because most people are terribly slow checkers relative to dedicated workers doing that job.