Size
I've got a Concept and I've found it to be good, if a little quirky.
The lining of the helmet wil compress over time to conform to your head shape, so while my helmet was initially tight to the point of giving me a headache, it is now slightly loose and earplugs are needed.
The visor works well and can be opened vertically and at either side. I thought it very noisy when I first used it but then found that the visor needs to be closed firmly at the sides - sudden peace. Gently clean the inside of the visor as you can remove the anti-fog coating. Open the visor slightly when in town or moving slowly - the C2 has an improved air flow inside the helmet but this is recommended. I have had my visor seal such that I required some effort to open it. Try opening it one side first to break the seal.