Incredibly low side effect rate, very effective, and a guarantee that you're going to get a mild version of the flu before everybody else does.
The reality is quite different.
The flu vaccine has to be produced several months before flu season. So, if the experts pick the wrong strains, or even if they pick the right ones and the flu mutates in that time, you're no better off.
In fact, you are worse off, as your immune system is likely to be worse off, trying to fight this new strain of the flu that is similar, but not exactly the same: http://en.wikipedia.org/wiki/Hoskins_effect
For the last 18 years getting a flu shot has been a federally mandated condition of my employment and I don't even work in a health care related field--what the heck is the big deal with getting a flu shot?
Freedom to make one's own decisions about medical treatment is a big thing in the US, and people dislike when they are compelled against their will. I'm no exception.
Just about all the improvements in public health over the past few centuries has been from an understanding of disease spread, and modern nutrition. Most people can and will like out the overwhelming majority of their lives without requiring any form of medical assistance. Being cognizant of the spread of the virus has a much higher success rate in preventing infection than does immunization.