Most days it isn't too bad as far as the customers go. Like any retail you will get the occasional bad customer. Before you even take a job at this company make sure you look at the location and get a good feel for how busy they actually are. If you get a busy place you can make some decent money. THE MANAGEMENT WILL LIE to you when you start about how many "book hours" you get if you are a mechanic. Which translates right into sales for the sale associates. A slower store is not even worth your time since they pay so low and most of your money comes from commissions/book hours depending if you work the shop or counter.
Most of the management is only concerned about "meeting weekly/daily sales" and less interested in meeting the needs of the customers. If something doesn't go a customer's way and they complain even the slightest the management will start knocking prices down to make them happy which will eat into and sales you make.
Management likes to hover a lot and micromanage everything.
Good luck ever getting a raise. Unless you change positions you will most likely not see a raise anytime soon. As a mechanic, you get raises with increased ASE certifications. I was there over 2.5 years and saw a $0.20 raise that entire time. If you work the counter count on what they give you to start, probably won't see much more.