I just spent some quick time using Apple Pay on an iPhone 6 Plus, and it's remarkably smooth. The phone was loaded up with a handful of credit cards, and holding the top of the phone about an inch from the card reader with my thumb on the TouchID home button triggered a payment from the default card in about five to 10 seconds. If you want to use a different card, just leave your finger off the home button and you'll see a Passbook-style card stack at the bottom of the screen, which you can tap on to bring up the full list and pick the card you want. Then you just put your finger on the reader and you're good to go.
Apple Pay also works with apps on the phone — the demo of choice here is Uber, which allows you to forego setting up an account if you want to use Apple Pay. Just hit "Ride Now" and you can call for a car by setting your location and authenticating against your Apple Pay account with Touch ID. It's super seamless, but there's no info on how much of a cut Apple's taking, so we'll see how many app developers want to use this system.