I have been wearing the Kask Protone for almost a year now and over the long term it has proven to be a great helmet. The first thing that you notice when you try on a Protone is the Octo Fit system. The adjustment it allows is outstanding an allows you to fit the helmet perfectly to you head and find the spot where the helmet is super comfy. The Protone is the successor to the wildly popular Mojito model and the Octo Fit is the evolution to it's Up Down System that you may be familiar with.
The Protone was developed to be an aero helmet that could be worn all day and still have enough airflow to keep your head cool. While I can't really tell much about the aero as i never seem to ride that fast, i can attest to it's airflow claims. Even on 6-8 hour rides in the 30+ heat the helmet was great and didn't cause in concern with overheating.
All in all this is a great helmet. With a RRP of $419 it certainly is a high end helmet. However with it's high end features it certainly is worth the premium price.