I have a KBC envy modular helmet, the chin bar is not completely removable but it opens up completely giving it a Transformers look and not protecting half of my face.
I crashed recently, not too violently but enough to crack every single part of the front fairing (some pieces are still somewhere on the road) and send my mirror flying.
I didn't know my head would have been smashed on the road until I looked at my helmet, and it stayed closed the whole time. I am now looking at a replacement helmet just in case mine took too big of a hit, it will most likely be one of the new Vemar (I love the look of the CKQI)