Well, here we go again. Not that I have anything new to say, it just feels wrong to fail to contribtue to a helmet thread having posted so often in its predecessors.
To summarize: please feel free to wear a helmet if you wish. If you're a mountainbiker tackling technical or challenging trails, definitely wear a full-face helmet. But if you're a roadie and expect the helmet to make a material difference in the event of a collision that might otherwise kill you, you're almost certainly mistaken.