It's a piece of cake to use and convenient. You can buy them at Best Buy, Apple Store, AT&T stores for $10, which you get refunded as soon as you activate your account. And now they have the option to take payments even if you don't have service, which is really convenient (orders are queued and processed as soon as you have service again). Also, if a card won't swipe, you can enter the CC number manually.
I use square on my iPad and on my iPhone.