Square seems to be the go-to AFAIK.. they probably have the widest following in terms of absolute number of businesses adopting them. I could be wrong though.
I've eaten at local places that used Square. I have no idea if they're happy customers.
I've seen increased use of Square at our local public farmers markets. The guy I buy my beef from loves his. It's lower initial cost, higher transaction cost, and super easy to use.
(I support any disruption that eats into the VISA/MasterCard/Etc oligarchy.)