http://www.treehugger.com/files/2008/09/helmet-debate-is-over.php

"If you just look at that, then the average of deaths pre-[legislation] and average number of deaths post-[legislation], there is a significant reduction. ... And it turns out it's a 52-per-cent reduction," said Patricia Parkin, senior author of the study and director of the Paediatric Outcomes Research Team at Toronto's Hospital for Sick Children.

I personally have been saved from head injury by wearing a bicycle helmet (http://ozreport.com/10.257#7). Recently I bumped my head pretty hard when two feet off the ground on a landing I was turned 180 degrees. I really appreciated having a nice helmet with a lot of foam.

There are arguments on both sides, but if wearing a helmet isn't getting in the way of you getting out and enjoying yourself on a bicycle, then put it on and go for it. This exercise (if you wish to think of it that way) is great for your cardio-vascular health and improves your leg strength for mountain launches.