In reply to JohnO'D:
I used to have a black diamond half dome, it was pretty basic, and got a bit sweaty as it didnt have any vents, however it did save my life when i fell off and smacked the back of my head on a rock, so i am happy to recommend them.
I now have a CAMP armour, its also quite cheap, and has a few vents and more comfortable padding than the old black diamond did, seems quite good up to now, but it hasnt had much action yet thankfully.