I had the opportunity to try out Square this evening while attending a Tech Crunch: Disrupt party at the NYC office of charity:water. In this picture below, I'm making a donation to charity:water on Jack Dorsey's iPhone as Jack watches.
I was quite impressed. The transaction was seamless and extremely quick. The receipt, which was emailed to me, includes a map of where the purchase was made. I could've received the receipt via SMS had I preferred that method.