My experience is that the assigned employee is always looking for something to do, because he can't leave the self-checkout area, but there isn't anything to actually do there. And well, the store better not accuse honest customers of anything, or else some stuff they really won't like will happen (and that applies to poor customers too).
Anyway, the experience is still so bad that I tend not to use it. But that's because the machines really suck.
I greatly prefer the single queue in self checkouts rather than betting on which cash register line will get stuck on someone that has a pricing issue or something. Obviously, this has nothing to do with self checkouts, but I find single queues far more ubiquitous after self checkouts came around than before.
For lots of stuff, a cashier is probably quicker. But I almost never have lots of stuff.
It's a classic false-positive problem. Most times when the self-checkout clerk has to give you attention, the problem is stupidly innocuous, so they blindly approve, as they have been trained by the system that it isn't a real problem.
I'm sure plenty of things get by them this way.
No such privilege is granted to regular customers. Instead, the self-checkout station locks itself up, and the customer has to wait several minutes for the assigned employee (who, most of the time, is also working two other tasks at the store) to show up, analyze the situation, enter service mode, and do the undo steps.