I've had a number of flu shots in the past, and I'd agree with those who said it feels like someone punched you in the arm and it goes away in a couple of hours.

However, I just got one yesterday, and my arm is still sore today. I've never had it last more than a day. A couple of my co-workers who also got it yesterday said it still hurts today, and we've all had plenty of flus shots in the past. There might be something different in the formulation this year, because it's significantly worse than normal.