Setting up the app
If you have a Starbucks gift card, launch the app and tap the "add new" button in the top right corner. You'll enter in your Starbucks card number and security code, and it will add the contents of that particular card to your account. Do the same for any other Starbucks cards that you own, then shred the physical cards. You don't need them any longer.
If you don't have a preloaded Starbucks card, there's no need to fret. First, create an account if you don't already have an older one with Starbucks. Then tap the account balance underneath the card picture and you get the option to "reload card," "refresh balance" and more options, which include setting up auto-reloading. Tap "reload card," enter your password, and you're taken to a screen where you can load your virtual Starbucks card in increments of $10. The first time you do a $25 reload on the card using a Visa-branded debit or credit card will net you $5 in extra credit. The app does store your credit card information, so if you're not using a passcode on your iPhone, now is the time to do so.
Once you've enabled payments on the app -- this can be toggled in the settings -- the "click to pay" button will appear on the app's home screen.
Using the app
After you have credit, head to the counter and order as normal. When it came time to pay, I tapped the "touch to pay" button then offered the barista my phone. She picked up a scanner that was a bit larger than my iPhone and aimed it at the barcode on the phone. It took her two tries, but it went through. "Your remaining balance is $19.82," she informed me just as the phone updated to reflect the remaining balance.
The app does a few other things. It keeps track of the My Starbucks Rewards program, which is Starbucks' frequent-customer program. You get a single star for installing and registering an account through the app. Five stars bumps you to Green Level, and 30 stars gets you Gold Status for a year. You get a star for each time you use the account, but refreshing it through the app did not give me a second credit for the purchase I made. That will take up to 24 hours.
I can see this quickly becoming a preferred method of utilizing a Starbucks card. It's one less physical card to have in your wallet, and it's quick to use. Setting it up for the first time takes no more than five minutes.
The biggest drawback is having your debit/credit card information stored in the app itself, though you can't reload a card without someone knowing your Starbucks password first. You can delete the card information off the app, but it's still worth taking the extra precaution and enabling the iPhone's built-in passcode just in case.