David wrote a review of Black Diamond Vapor Helmet on July 2, 2013
Familiarity: I've used it once or twice and have initial impressions
The helmet is light weight. So light it is easy to forget you are wearing it. The ventilation is superb. My head stayed relatively cool on a hot day of slide climbing in the Adirondack mountains.
Sizing: My head measured 23 inches in diameter and I ordered both sizes the S/M and the M/L since the high end of the S/M was 23 and the low end of the M/L was also 23. I found that for my head the S/M was much more comfortable. The ratchet adjustment in the back of the helmet helped to customize the fit and allow the helmet to sit securely on my head.