The funny thing is, most of the supermarkets use exactly the same system. I know Tesco, M&S and Asda have the exact same system just rebranded. Equally, I think Morrisons use a differeny companies system. I've heard far more problems with Morrison's from a friend who works there than I've experienced with the Tesco ones where I'm being trained.
If its quiet, self serve is generally much faster as there isn't usually a queue. But if you have alcohol/age restricted items then depending on how busy it is, you may be faster going through regular tills, depends if the person watching the self serve is paying attention or not.
That said, you do get some real idiots. Had a customer on saturday who was just buying a newspaper. Scanned it then just folded it up under his arm, the checkout kept telling him to put it in the bagging area, so he stormed off to one of the basket tills complaining about how it wasn't letting him pay.