geez didn't think I would raise a debate but ok, I happen to be a medical researcher (not practitionner, can't stand sick people
one scientific study published last year surveyed 1,205 cases of seasonal influenza amongst military personnel stationed in the U.S. Results show that the vaccine (either TIV or LAIV type) was only marginally effective in preventing mild symptoms, but was markedly effective in preventing severe symptoms.
so here's the verdict: if you could be getting real badly sick (e.g., you are immuno-compromised, e.g., HIV-positive, over the age of 65, etc.) then probably best to take it to avoid potentially severe symptoms. The rest of us (like me), we usually bounce back within a few days.
here's the study in question:
Seasonal influenza vaccine and protection against ... [PLoS One. 2010] - PubMed result
you can find many more studies if you search the pubmed database, but, as always, read with a critical eye as definitive answers are rare.