My local coffee shop here in Tempe, AZ switched over to using an iPad with Square instead of a cash register about a month ago.
As a customer I like it. It's fast and slick. The employees seem to like it too.
Only one thing: if anybody from Square is reading this look into allowing the option of a bigger, sturdier card reader. The little on you ship now is flimsy and doesn't work too well if you use it 500 times a day.