Danny
All depended on the shape of your head! There was a Sikh corporal at North Luffenham who fitted me with my first helmet, a Mk. 3 C with dual visors in 1979. The only problem was that he insisted on giving me a Medium one.
It was agony! After only a short time I had a splitting headache. Happily I was re-issued a Medium Broad and it was fine. The only gripe was its weight. Under high g it was a pain, literally. This I think was why some pilots (certainly in the Air Defence world) preferred using their old Mk. 1A's with cloth inner......
Another downside (at least for the Mk.3) was that the ear seals were filled with fluid rather than foam, supposedly for better hearing protection. I had a seal burst and it was like having a nosebleed.
The real advantage of any helmet though is the birdstrike/wind blast protection the visors are supposed to offer and (in a dual control aircraft) when the hands-on pilot makes a sudden aileron input you find your head "lagging" the roll and hitting the side of the canopy! Without the helmet it could hurt.
MB