Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

There are going to be occasions where you might as well have have your head wrapped in damp lettuce rather than a helmet for all the good it is going to do you. If you're unlucky enough to end up in one of those knocks I think its more important to be wearing decent pants than anything else so you don't embarrass yourself too much in front of the nurses. For everything under this threshold the helmet can be the difference between serious head injury and a simple road rash - a woman got killed some years back in Paddington because some little shits snatched her bag while she was riding along, she fell off and hit her head on the kerb sans helmet and died as a direct result. As a matter of fact you do look like a knob with a lid on but I'd hate to die of some poxy fractured skull resulting from a sticky cleat at the lights at zero mph.