Don't have them @ our local Costco yet (Roseville) but do @ Home Depot & Albertson's. Takes one attendant for 4 registers @ both stores. Typical customer using it has a handful of items. The bag area weighs each item & knows from the SKU what it should weigh so it "knows" if you put the scanned item in the bag or some other item (gotta put it in the bag to go to the next scan). Works great. Most customers need a quick lesson their first time, then only when there's a problem (bad bar code,...). Won't work well w/coupons or other less-than-straight-forward purchases; won't work well w/cash till bill scanners get better @ not rejecting good bills.
This will become the norm in the future. Not a source of competitive advantage IMO; everybody in retail can do this to some extent (mom w/kids can't do self-checking or @ least won't).
I thought this would be limited when I first saw it, but it will ramp up quickily in next 2 years as customers like it more & more. Its nice not waiting on some zit-faced kid-checker blabbing to her freind running the next register; now I can fumble thru @ my own pace waiting only on me.