Also keep in mind that helmets do have expiration dates. It is good practice to replace helmets every 3-5 years. Most helmets are good for 5 when purchasing a current model year. Watch out for clearance helmets because they have usually been on a shelf for a few years.
Reason for expiration date is the glue and fastening materials degrade over time. Impact protection is reduced as a result.
Safe bet IMHO is anything new over $200 from a major manufacturer is good for cruising. If you are going to do serious spirited riding or track days, invest in a high quality race level helmet.
All helmets might meet the DOT or SNELL standard, but remember, the top of the line helmets far exceed the standards. There are no numbers that I know of that show the difference in testing of meeting or exceeding. Just pass or fail. So there are big differences in cheap approved helmets and the expensive ones.
Final thought and most important: best helmet is the one that fits. If the helmet moves while wearing it, it wont protect you properly. Fit is everything, and higher quality helmets come with replaceable cheek pads of various sizes to ensure best fit and protection.
I wear ARAI and Scorpion and find they fit me well. I have more confidence in the ARAI. SHOEI is also a well respected brand.
Take some time to watch these videos and really listen, great info that could save your life and those riding with you.
Some great videos that explain this topic on youtube:
http://www.youtube.com/watch?v=rRmWBp65gbohttp://www.youtube.com/watch?v=Fyvpvcs-ixI&feature=related