FYI- All helmets sold in the USA have to pass the same safety standards.

This means that a $16 Wal-Mart helmet will protect your brain just as well as a $250 racing helmet. The differences between the 2 helmets are primarily 3 things:

• Name brand
• # of vents
• Weight

The more expensive a helmet is (as a general rule) the lighter it will be and the more vents it will have. Havign a helmet that is comfortable and keeps your head cool in the summer is important because if it isn't comfortable you are less likely to wear it.
I've always liked the Gyro Hex because it looks cool. I'm about to replace my helmet and I may go with a much dorkier looking Bell Metro for the winter and a Bell Influx (or something similar) for the summer.

Helmets should be replaced immediately after any falls/crashes where they hit the ground [even if they don't look damaged from the outside] or every 5 or 6 years if you remain crash-free.