Some times I'm curious to see how stores work at the US nowadays.
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.
Between Costco/Target/Winco/Walmart/Home Depot/Lowes/Kroger/Uniqlo, my experience is that I can check out quicker than before. I rarely have to wait for assistance, which itself is rarely needed.
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.