When I worked at Radioshack as an employee and a manager, I thought it was really awkward when a customer would call me my name out of the blue. Sometimes I suspect they did it because it caused a pause in the conversation - especially if I was answering their questions about a product - and wanted to stop the close.
But I think the most discomforting thing would be the regulars who'd come in and walk up and go 'Hey x, blah blah blah blah," like we are the best of buddies and them using my first name makes me want to help them with more enthusiasm than - you are my customer, here's your shit, now get out. With a smile.
On the other hand I do a lot of phone work now and I don't mind my name being used so much because I will use someone's name to the point of uncomfortability, interjecting it at every opportunity. Because I can.