I've seen the term "USian" a lot but I've very rarely heard it spoken. When it was it was pronounced as if you say the letters U-S-E-N. I agree with others that I don't see it as particularly offensive.
Now, for my SS story of the day, I was at the grocery in the express line buying several items but visibly well below the limit for the lane. I unloaded my stuff on the belt, and then someone stepped up beside me (the express lane didn't have anything to one side of it so he approached directly from the side) and put a loaf of bread on top of my stuff on the belt. His excuse when I asked him why he did that was, "I'm in a hurry." When the cashier put his item aside, he started to grouse until she told him he'd have to wait or leave.