I'd get a new one.
Based on the fact that your head is pretty important - I mean you can even live without a limb but not without your head.
I agree with Graeme about different opinions.
I recently had a nail in my tyre.
Loads of people said repair it, some said no way.
In the end I knew I'd be happier replacing it rather then worrying about it all the time, so I replaced it, but plenty of people would have repaired it.
I'm probably risk averse, after all in 14 years of riding I've never had to rely on a helmet - but then you never know when there will be a first time.
But overall "a hell of a crack" would indicate to me that something broke and they are only designed to work once.
