If you're getting it done, aim to get it by Easter (ish) each year. Any later than that and flu season will already have hit, increasing the chance that you could be sick by the time you get the needle, kwim?

I've had it every year for the last 7 years and have never had any problems with it. They change the strain every year so over time (in theory) you're immune to more and more of the (nastier) strains in current circulation.