The fit of the helmet is important, it should be tight enough that the pads firmly touch your cheeks. It should be tight enough that you can't grab it and roll it off or rotate it from side to side very much. But, it should be loose enough that it doesn't pinch or bind your cheeks or forehead, this will cause fatigue. Sizes vary by brand, A large in one brand will be like a medium in another brand. You must try on the exact brand/model in a new (not used) version before buying. Go to a dealer or cycle shop and try on several, then order mail-order. (and buy something small from the dealer for his/her trouble). If it comes in the mail and doesn't fit right, return it!
Some people claim that helmets restrict your vision, it's not true. Others claim it reduces your hearing, it actually helps you hear over the engine. As for the claim they cause neck injuries, I can't offer any evidence on that but I will take my chances. Wear one for a month and decide for yourself. The first week it will feel wierd (especially a full face) but it will quickly become as comfy and familiar as your old fuzzy blanket.
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