I don't want this to be a debate about helmet laws. This is for people who are wearing a helmet, regardless of reason.
A couple weeks ago I did a 3-day charity ride and noticed that the majority (70%+) of riders had Giro helmets. Aside from looking pretty cool, Giro claims to have the most aerodynamic all-purpose helmets on the road while still providing good ventilation.
I'm riding in my $20 Schwinn helmet for really one reason: safety. I've had quite a few times where it's saved me from injury (mostly from tree branches lower than I thought).
Seeing so many people with $100-$200 helmets made me wonder: What about it makes it worth it? Sure, the pros use them, but they are also sponsored to use them. Are they actually effective in shaving off time on a ride?
Generally, I ride casusally, and giving myself shortcuts will only mean longer distances without any real effort gain. But, then again, I'd like to be able to say that I can easily handle 50 miles a day.
I'm getting off topic, so i'll repose my question: What difference, performance-wise, does a good helmet make?