My "gripe" is with the ease of fraud with this device.
I think that will be the concern of most people; rampant identify theft and credit card fraud will make them think twice about giving their credit card to someone with a little square white dongle haphazardly attached to their phone. The rest will get concerned when they get fleeced, and no sooner.
The quid pro quo seems obvious here. Square limits the amount they will let you charge. The person accepting a card doesn't really know if that card is legit or the user is a deadbeat about to go bankrupt. If Square doesn't get the money from the credit card user, they are going to charge back to the acceptor, that is you. (I know, it really isn't a charge back, it will simply be no cash collected. It will be a sale with nothing in return.)
When you use a credit card, you risk someone getting your credit card information. The credit card companies know this as well and limit the liability of a card holder. If someone tries to use your credit card fraudulently (by stealing your credit card information), it is the credit card company and the merchant (you) that are held liable for losses. If it wasn't done that way, credit card usage would fall off greatly.
I'm going to look into getting one too.