I've spent the day shopping with Pay and an Watch.
I've used it at Waitrose, LIDL, Asda, Tesco and paid for my London bus fare with the Watch, all without a hitch and without delay at the terminal.
It is definitely faster and easier than fumbling into your pocket or handbag and then into your wallet or purse to find a card.
From my experience today Pay may well be the killer app for the Watch.
It makes it a joy to pay!