SanSFiS (i.e. the parent of Conjecture and Westercon 68) has been using Square for several years for both fan table sales, and at the door sales. I don't know if we were using it the last time we were officially in charge of Anime Conji, so we may have only used it on Conjecture/Conchord sized events (~300 total maybe 100 at the door).

The application has been getting better with each release - at least once you poke deep enough to find all of the neat things you can do. I've actually been (trying to) use it to record even the cash and check sales at fan tables to give us yet one more record of the transaction.