Flu can make you very ill. Several years ago I caught the flu over the Xmas holiday from a family member. He was ill, but only for a week or so, but I was extremely ill for six weeks. At the time I was self employed and had deadlines to meet so I did try to drag myself out of bed to do some work, inevitably collapsing in a short time. I lost work and quite a lot of money by being so ill.
The following year I made sure I got a flu jab, although I was at the time a year or two younger than the age when you are offered the jab automatically. I have had one every year since then and have never been ill. You are not guaranteed protection as their are many strains of flu and the jab each year is designed to protect against the strains which are most prevelant, but even so, I would advise everyone to have a flu jab unless advised not to by their doctor.