Some points to consider: Already mentioned is protection, if it has been dropped (from as little as 4 feet), it may have lost it's protective abilities....main reason not to leave your helmet on a bike seat....if it has been dropped, replace it.
The other is fit; For whatever reason, I find HJC helmets fit my head. I own two...and a poor fitting helmet does not give the protection. Can you buy via the internet? Sure...but, remember...fit is crucial. And yeah, sure...go to the local bike shop and try them on...then buy on the internet...but!....the guys in the bike shop need to make a living too and you're paying a premium for the fitting and sales help, which is hard to get over the internet I also think we owe it to local guys to do some local buying).
I have a HJC Sy-max II.....great protection, fit and ventilation. And too darn loud. The wind noise is so loud, I go back to my HJC half helmet.....which brings me to hearing protection. I've tried bunches and my left ear canal is larger than my right, and I haven't yet found anything to last a day (either they are uncomfortable, like the ones I had an audiologist make for me) or they don't effectively reduce the noise...and don't sell hearing short.
Sorry for being pedantic...part of my background running MSF courses.