The best self-checkout system I've encountered are the ones they have at Sams Club (and presumably at similar warehouse clubs like Costco and BJs). There you swipe or insert your membership card. Once it has processed that, you can take the hand-held scanner and scan all the items in your cart (when putting things in the cart, I'll make sure the UPC codes are exposed).

Once you're done scanning, you place the scanner back into its holder, choose your payment option, and make your payment. The only other thing you need to do is have someone check your receipt before you exit the store.

It would be nice if those types of machines were in more stores as opposed to the ones that require you to take items out of the cart and either scan or weigh/enter code and then bag them.