I've already gotten my flu shot. I have asthma so that is a compelling enough reason.
When I was a young adult I didn't get the flu shot because it didn't cross my mind as something I needed. But then I got sick one holiday after my nephew, who was sick, coughed in my face and I felt spit land in my eye (gross). One day later, I was down and out for a whole week...fever, aches, skin sensitivity. I missed a week visiting with my relatives.
After that I started getting flu shots and haven't gotten the flu since. I'd rather take the discomfort of the shot and have a better chance of not getting the flu. My shot side effects are a sore arm and sometimes feel a little tenderness in the lymph nodes under my armpit the next day.
The problem with flu shots is that you never know if you would have gotten the flu if you hadn't gotten the shot. So it's hard to say that the flu shot did its job or not. All you know is that you didn't get the flu.
I have kids now and I'm more on top of controlling my asthma so it's worth it to me. Even if I didn't get it free through my employer, I'd still go get one on my own.