I measured my head according to manufactures chart and ordered the medium which was just barely bigger than my had according to the measurements. When I tried it on it was way to tight. I sent it back, which was painless, and got the large. As someone else stated they do run small.
Overall the first helmet looked pretty abused as maybe several others ordered that size and it didn't fit either. The graphics where beginning to chip off mostly on the front mouth guard. The second one looks much nicer and the plastic bag it came in was actually tied around the base of the helmet like it never came out of the box. Much happier with my decision to return the other and go up a size. I have not had a chance to go riding with it yet but I'm sure it will perform well like any other helmet.
Not even kidding, this is the best helmet I have ever bought and my last few helmets were much more expensive Arai XD and Shoei dualsport helmets! I could not be happier with these. Everything about them is just great! I bought 6 of these in different colors for the price of one Arai. Try one on!
I have only used this helmet a few times but I am pleased with the purchase. It fits well and is comfortable. It seems to breathe okay but I admit, I haven't ridden on a really warm day yet. I think it's a good helmet for the money. I would buy one again.