Getting immunized is worth it. Even if it doesn’t work as thoroughly as expected, 57% success is still a helluva lot better efficacy than “rub some dirt in it, and walk it off”.
As, mentioned, even if you do get the flu, the vaccination can reduce the illness’s severity and duration. There’s also value in herd immunity. Even if the vaccine doesn’t get the most prevalent strain, by vaccinating you’re helping to protect infants, the elderly, people with allergies to eggs (and other components of the flu shot) and immune compromised people from the strains the vaccine does protect for.
The side-effects are very minimal. You might get a little bit of bruising, but probably won’t if the nurse administering has any experience. You could get a little bit of a rash, although that’s totally worth not being sick for a week.
Rarely there are severe adverse effects. But if you were getting regularly vaccinated, you would have already become aware of it.