I just did a review for C&K's WW Annual on helmets and received quite the education.

Many of the plastic helmets just do not provide adequete protection - the shells are just too soft and flex too much.  NRS staff even went so far as to tell me that they didnt recommend plastic shells for anything above Class II.  Those that use polycarbonates - which are stiffer - are the exception.  A dual density liner also seems to be mandatory for serious use.

I had a rude awakening last year and came out of it with a cracked Rokk Heads Helmet, a concussion and two compressed vertebrae all in a class II rapid.  Up until that day helmets were just something that I wore to protect from the occasional scrape.  That event opened my eyes big.

The review should be on newsstands now.