Each year the World Health Organization forecasts which flu strains are likley to be predominant in the Northern Hemisphere during flu season. Its a bit like rolling dice as predicting virus behaviour is a very inexact science. in a good year protection can be 60% or higher. Last year the vaccine offered almost zero protection.
This year early signs are H3N2 will be the predominant strain. Testing by the National Microbiology Laboratory is showing so far that the vaccine is a good match to the H1N1 and Influenza B strains but not the H3N2 one. So, estimates are that this year's flu shot will be 30-50% effective.
Should you still get the shot? Well, if you could reduce your chances of getting a really nasty illness by up to 50% would you take it? I would!