The upside of receiving the flu vaccine is that it offers some protection against contracting the flu - not to be confused, as a lot of people do, with the common cold. The level of protection it offers varies depending on the individual and their immune system, and the match between the vaccine profile and the strain of flu circulating at the time - always a little bit of a lottery.
If its a good match, and you have a reasonable immune system, then you should benefit from the vaccine.
Downsides - localised soreness, redness. systemically, you might suffer a slight temperature - and err, thats about it.
On balance, if in your life you encounter lots of people on a daily basis, then its probably worth having.
Just do not forget that basic good hygiene practices can offer good protection against contracting flu, especially washing your hands.....