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.
The Square register was a huge flop with most merchants (though they are addressing these issues with more recent updates). Most merchants I've interviewed that use an iPad as a POS solution have tried it, and dumped it for something that actually met their needs. Again, they are addressing these issues, but that was the feedback I received less than 6 months ago.
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.)
Sorry, I don't have much info other than it didn't have the critical features they needed in order for it to be useful. The details of those features I never asked about. What I can tell you is that I mainly spoke with cafes, restaurants and other food service related merchants. Only spoke with a handful of retail merchants... Hope that helps!